[2/2] xen/mm: Introduce PGC_state_uninitialised

Commit Message

David Woodhouse March 19, 2020, 9:21 p.m. UTC

From: David Woodhouse <[e-mail]>

It is possible for pages to enter general circulation without ever
being process by init_heap_pages().

For example, pages of the multiboot module containing the initramfs may
be assigned via assign_pages() to dom0 as it is created. And some code
including map_pages_to_xen() has checks on 'system_state' to determine
whether to use the boot or the heap allocator, but it seems impossible
to prove that pages allocated by the boot allocator are not subsequently
freed with free_heap_pages().

This actually works fine in the majority of cases; there are only a few
esoteric corner cases which init_heap_pages() handles before handing the
page range off to free_heap_pages():
 • Excluding MFN #0 to avoid inappropriate cross-zone merging.
 • Ensuring that the node information structures exist, when the first
   page(s) of a given node are handled.
 • High order allocations crossing from one node to another.

To handle this case, shift PG_state_inuse from its current value of
zero, to another value. Use zero, which is the initial state of the
entire frame table, as PG_state_uninitialised.

Fix a couple of assertions which were assuming that PG_state_inuse is
zero, and make them cope with the PG_state_uninitialised case too where
appopriate.

Finally, make free_heap_pages() call through to init_heap_pages() when
given a page range which has not been initialised. This cannot keep
recursing because init_heap_pages() will set each page state to
PGC_state_inuse before passing it back to free_heap_pages() for the
second time.

Signed-off-by: David Woodhouse <[e-mail]>
---
 xen/arch/x86/mm.c        |  3 ++-
 xen/common/page_alloc.c  | 44 +++++++++++++++++++++++++++++-----------
 xen/include/asm-arm/mm.h |  3 ++-
 xen/include/asm-x86/mm.h |  3 ++-
 4 files changed, 38 insertions(+), 15 deletions(-)

Comments

Paul Durrant March 20, 2020, 1:33 p.m. UTC | #1

> -----Original Message-----
> From: Xen-devel <[e-mail]> On Behalf Of David Woodhouse
> Sent: 19 March 2020 21:22
> To: [e-mail]
> Cc: Stefano Stabellini <[e-mail]>; Julien Grall <[e-mail]>; Wei Liu <[e-mail]>;
> Andrew Cooper <[e-mail]>; Ian Jackson <[e-mail]>; George Dunlap
> <[e-mail]>; [e-mail]; Jan Beulich <[e-mail]>; Volodymyr Babchuk
> <[e-mail]>; Roger Pau Monné <[e-mail]>
> Subject: [Xen-devel] [PATCH 2/2] xen/mm: Introduce PGC_state_uninitialised
> 
> From: David Woodhouse <[e-mail]>
> 
> It is possible for pages to enter general circulation without ever
> being process by init_heap_pages().
> 
> For example, pages of the multiboot module containing the initramfs may
> be assigned via assign_pages() to dom0 as it is created. And some code
> including map_pages_to_xen() has checks on 'system_state' to determine
> whether to use the boot or the heap allocator, but it seems impossible
> to prove that pages allocated by the boot allocator are not subsequently
> freed with free_heap_pages().
> 
> This actually works fine in the majority of cases; there are only a few
> esoteric corner cases which init_heap_pages() handles before handing the
> page range off to free_heap_pages():
>  • Excluding MFN #0 to avoid inappropriate cross-zone merging.
>  • Ensuring that the node information structures exist, when the first
>    page(s) of a given node are handled.
>  • High order allocations crossing from one node to another.
> 
> To handle this case, shift PG_state_inuse from its current value of
> zero, to another value. Use zero, which is the initial state of the
> entire frame table, as PG_state_uninitialised.
> 
> Fix a couple of assertions which were assuming that PG_state_inuse is
> zero, and make them cope with the PG_state_uninitialised case too where
> appopriate.
> 
> Finally, make free_heap_pages() call through to init_heap_pages() when
> given a page range which has not been initialised. This cannot keep
> recursing because init_heap_pages() will set each page state to
> PGC_state_inuse before passing it back to free_heap_pages() for the
> second time.
> 
> Signed-off-by: David Woodhouse <[e-mail]>
> ---
>  xen/arch/x86/mm.c        |  3 ++-
>  xen/common/page_alloc.c  | 44 +++++++++++++++++++++++++++++-----------
>  xen/include/asm-arm/mm.h |  3 ++-
>  xen/include/asm-x86/mm.h |  3 ++-
>  4 files changed, 38 insertions(+), 15 deletions(-)
> 
> diff --git a/xen/arch/x86/mm.c b/xen/arch/x86/mm.c
> index 62507ca651..5f0581c072 100644
> --- a/xen/arch/x86/mm.c
> +++ b/xen/arch/x86/mm.c
> @@ -491,7 +491,8 @@ void share_xen_page_with_guest(struct page_info *page, struct domain *d,
> 
>      page_set_owner(page, d);
>      smp_wmb(); /* install valid domain ptr before updating refcnt. */
> -    ASSERT((page->count_info & ~PGC_xen_heap) == 0);
> +    ASSERT((page->count_info & ~PGC_xen_heap) == PGC_state_inuse ||
> +           (page->count_info & ~PGC_xen_heap) == PGC_state_uninitialised);

Could the page state perhaps be bumped to inuse in this case? It seems odd to leave state uninitialized yet succeed in sharing with a guest.

> 
>      /* Only add to the allocation list if the domain isn't dying. */
>      if ( !d->is_dying )
> diff --git a/xen/common/page_alloc.c b/xen/common/page_alloc.c
> index 8d72a64f4e..4f7971f2a1 100644
> --- a/xen/common/page_alloc.c
> +++ b/xen/common/page_alloc.c
> @@ -252,6 +252,8 @@ struct bootmem_region {
>  static struct bootmem_region __initdata
>      bootmem_region_list[PAGE_SIZE / sizeof(struct bootmem_region)];
>  static unsigned int __initdata nr_bootmem_regions;
> +static void init_heap_pages(struct page_info *pg, unsigned long nr_pages,
> +                            bool scrub);
> 
>  struct scrub_region {
>      unsigned long offset;
> @@ -1390,6 +1392,17 @@ static void free_heap_pages(
>      ASSERT(order <= MAX_ORDER);
>      ASSERT(node >= 0);
> 
> +    if ( page_state_is(pg, uninitialised) )
> +    {
> +        init_heap_pages(pg, 1 << order, need_scrub);
> +        /*
> +         * init_heap_pages() will call back into free_heap_pages() for
> +         * each page but cannot keep recursing because each page will
> +         * be set to PGC_state_inuse first.
> +         */
> +        return;
> +    }
> +
>      spin_lock(&heap_lock);
> 
>      for ( i = 0; i < (1 << order); i++ )
> @@ -1771,11 +1784,10 @@ int query_page_offline(mfn_t mfn, uint32_t *status)
>   * latter is not on a MAX_ORDER boundary, then we reserve the page by
>   * not freeing it to the buddy allocator.
>   */
> -static void init_heap_pages(
> -    struct page_info *pg, unsigned long nr_pages)
> +static void init_heap_pages(struct page_info *pg, unsigned long nr_pages,
> +                            bool scrub)
>  {
>      unsigned long i;
> -    bool idle_scrub = false;
> 
>      /*
>       * Keep MFN 0 away from the buddy allocator to avoid crossing zone
> @@ -1800,7 +1812,7 @@ static void init_heap_pages(
>      spin_unlock(&heap_lock);
> 
>      if ( system_state < SYS_STATE_active && opt_bootscrub == BOOTSCRUB_IDLE )
> -        idle_scrub = true;
> +        scrub = true;
> 
>      for ( i = 0; i < nr_pages; i++ )
>      {
> @@ -1828,7 +1840,8 @@ static void init_heap_pages(
>              nr_pages -= n;
>          }
> 
> -        free_heap_pages(pg + i, 0, scrub_debug || idle_scrub);

Would it be worth an ASSERT(!pg[i].count_info) here in case something is added which erroneously modifies the page count info before this is done?

> +        pg[i].count_info = PGC_state_inuse;
> +        free_heap_pages(pg + i, 0, scrub_debug || scrub);
>      }
>  }
> 
> @@ -1864,7 +1877,7 @@ void __init end_boot_allocator(void)
>          if ( (r->s < r->e) &&
>               (phys_to_nid(pfn_to_paddr(r->s)) == cpu_to_node(0)) )
>          {
> -            init_heap_pages(mfn_to_page(_mfn(r->s)), r->e - r->s);
> +            init_heap_pages(mfn_to_page(_mfn(r->s)), r->e - r->s, false);
>              r->e = r->s;
>              break;
>          }
> @@ -1873,7 +1886,7 @@ void __init end_boot_allocator(void)
>      {
>          struct bootmem_region *r = &bootmem_region_list[i];
>          if ( r->s < r->e )
> -            init_heap_pages(mfn_to_page(_mfn(r->s)), r->e - r->s);
> +            init_heap_pages(mfn_to_page(_mfn(r->s)), r->e - r->s, false);
>      }
>      nr_bootmem_regions = 0;
> 
> @@ -2142,7 +2155,7 @@ void init_xenheap_pages(paddr_t ps, paddr_t pe)
> 
>      memguard_guard_range(maddr_to_virt(ps), pe - ps);
> 
> -    init_heap_pages(maddr_to_page(ps), (pe - ps) >> PAGE_SHIFT);
> +    init_heap_pages(maddr_to_page(ps), (pe - ps) >> PAGE_SHIFT, false);
>  }
> 
> 
> @@ -2251,7 +2264,7 @@ void init_domheap_pages(paddr_t ps, paddr_t pe)
>      if ( mfn_x(emfn) <= mfn_x(smfn) )
>          return;
> 
> -    init_heap_pages(mfn_to_page(smfn), mfn_x(emfn) - mfn_x(smfn));
> +    init_heap_pages(mfn_to_page(smfn), mfn_x(emfn) - mfn_x(smfn), false);
>  }
> 
> 
> @@ -2280,7 +2293,8 @@ int assign_pages(
> 
>          for ( i = 0; i < (1ul << order); i++ )
>          {
> -            ASSERT(!(pg[i].count_info & ~PGC_extra));
> +            ASSERT((pg[i].count_info & ~PGC_extra) == PGC_state_inuse ||
> +                   (pg[i].count_info & ~PGC_extra) == PGC_state_uninitialised);

Again, perhaps bump the state to inuse if it is uninitialized...

>              if ( pg[i].count_info & PGC_extra )
>                  extra_pages++;
>          }
> @@ -2316,10 +2330,16 @@ int assign_pages(
>      for ( i = 0; i < (1 << order); i++ )
>      {
>          ASSERT(page_get_owner(&pg[i]) == NULL);
> +        /*
> +         * Note: Not using page_state_is() here. The ASSERT requires that
> +         * all other bits in count_info are zero, in addition to PGC_state
> +         * being appropriate.
> +         */
> +        ASSERT((pg[i].count_info & ~PGC_extra) == PGC_state_inuse ||
> +               (pg[i].count_info & ~PGC_extra) == PGC_state_uninitialised);

...then this ASSERT can be tightened.

>          page_set_owner(&pg[i], d);
>          smp_wmb(); /* Domain pointer must be visible before updating refcnt. */
> -        pg[i].count_info =
> -            (pg[i].count_info & PGC_extra) | PGC_allocated | 1;
> +        pg[i].count_info = (pg[i].count_info & PGC_state) | PGC_allocated | 1;

The PGC_extra seems to have vapourized here.

  Paul

>          page_list_add_tail(&pg[i], &d->page_list);
>      }
> 
> diff --git a/xen/include/asm-arm/mm.h b/xen/include/asm-arm/mm.h
> index a877791d1c..49663fa98a 100644
> --- a/xen/include/asm-arm/mm.h
> +++ b/xen/include/asm-arm/mm.h
> @@ -113,12 +113,13 @@ struct page_info
>    * { inuse, offlining, offlined, free, broken_offlining, broken }
>    */
>  #define PGC_state                  PG_mask(7, 9)
> -#define PGC_state_inuse            PG_mask(0, 9)
> +#define PGC_state_uninitialised    PG_mask(0, 9)
>  #define PGC_state_offlining        PG_mask(1, 9)
>  #define PGC_state_offlined         PG_mask(2, 9)
>  #define PGC_state_free             PG_mask(3, 9)
>  #define PGC_state_broken_offlining PG_mask(4, 9) /* Broken and offlining */
>  #define PGC_state_broken           PG_mask(5, 9) /* Broken and offlined */
> +#define PGC_state_inuse            PG_mask(6, 9)
> 
>  #define pgc_is(pgc, st)            (((pgc) & PGC_state) == PGC_state_##st)
>  #define page_state_is(pg, st)       pgc_is((pg)->count_info, st)
> diff --git a/xen/include/asm-x86/mm.h b/xen/include/asm-x86/mm.h
> index 1203f1b179..5fbbca5f05 100644
> --- a/xen/include/asm-x86/mm.h
> +++ b/xen/include/asm-x86/mm.h
> @@ -72,12 +72,13 @@
>    * { inuse, offlining, offlined, free, broken_offlining, broken }
>    */
>  #define PGC_state                  PG_mask(7, 9)
> -#define PGC_state_inuse            PG_mask(0, 9)
> +#define PGC_state_uninitialised    PG_mask(0, 9)
>  #define PGC_state_offlining        PG_mask(1, 9)
>  #define PGC_state_offlined         PG_mask(2, 9)
>  #define PGC_state_free             PG_mask(3, 9)
>  #define PGC_state_broken_offlining PG_mask(4, 9) /* Broken and offlining */
>  #define PGC_state_broken           PG_mask(5, 9) /* Broken and offlined */
> +#define PGC_state_inuse            PG_mask(6, 9)
> 
>  #define pgc_is(pgc, st)            (((pgc) & PGC_state) == PGC_state_##st)
>  #define page_state_is(pg, st)       pgc_is((pg)->count_info, st)
> --
> 2.21.0
> 
> 
> _______________________________________________
> Xen-devel mailing list
> [e-mail]
> https://lists.xenproject.org/mailman/listinfo/xen-devel

Jan Beulich March 20, 2020, 1:53 p.m. UTC | #2

On 20.03.2020 14:33, Paul Durrant wrote:
>> -----Original Message-----
>> From: Xen-devel <[e-mail]> On Behalf Of David Woodhouse
>> Sent: 19 March 2020 21:22
>>
>> --- a/xen/arch/x86/mm.c
>> +++ b/xen/arch/x86/mm.c
>> @@ -491,7 +491,8 @@ void share_xen_page_with_guest(struct page_info *page, struct domain *d,
>>
>>      page_set_owner(page, d);
>>      smp_wmb(); /* install valid domain ptr before updating refcnt. */
>> -    ASSERT((page->count_info & ~PGC_xen_heap) == 0);
>> +    ASSERT((page->count_info & ~PGC_xen_heap) == PGC_state_inuse ||
>> +           (page->count_info & ~PGC_xen_heap) == PGC_state_uninitialised);
> 
> Could the page state perhaps be bumped to inuse in this case? It
> seems odd to leave state uninitialized yet succeed in sharing with a guest.

This would be quite nice indeed, if of course it doesn't cause
new complications.

Jan

David Woodhouse March 20, 2020, 3:17 p.m. UTC | #3

On Fri, 2020-03-20 at 13:33 +0000, Paul Durrant wrote:
> > -----Original Message-----
> > From: Xen-devel <[e-mail]> On Behalf Of David Woodhouse
> > Sent: 19 March 2020 21:22
> > To: [e-mail]
> > Cc: Stefano Stabellini <[e-mail]>; Julien Grall <[e-mail]>; Wei Liu <[e-mail]>;
> > Andrew Cooper <[e-mail]>; Ian Jackson <[e-mail]>; George Dunlap
> > <[e-mail]>; [e-mail]; Jan Beulich <[e-mail]>; Volodymyr Babchuk
> > <[e-mail]>; Roger Pau Monné <[e-mail]>
> > Subject: [Xen-devel] [PATCH 2/2] xen/mm: Introduce PGC_state_uninitialised
> > 
> > From: David Woodhouse <[e-mail]>
> > 
> > It is possible for pages to enter general circulation without ever
> > being process by init_heap_pages().
> > 
> > For example, pages of the multiboot module containing the initramfs may
> > be assigned via assign_pages() to dom0 as it is created. And some code
> > including map_pages_to_xen() has checks on 'system_state' to determine
> > whether to use the boot or the heap allocator, but it seems impossible
> > to prove that pages allocated by the boot allocator are not subsequently
> > freed with free_heap_pages().
> > 
> > This actually works fine in the majority of cases; there are only a few
> > esoteric corner cases which init_heap_pages() handles before handing the
> > page range off to free_heap_pages():
> >  • Excluding MFN #0 to avoid inappropriate cross-zone merging.
> >  • Ensuring that the node information structures exist, when the first
> >    page(s) of a given node are handled.
> >  • High order allocations crossing from one node to another.
> > 
> > To handle this case, shift PG_state_inuse from its current value of
> > zero, to another value. Use zero, which is the initial state of the
> > entire frame table, as PG_state_uninitialised.
> > 
> > Fix a couple of assertions which were assuming that PG_state_inuse is
> > zero, and make them cope with the PG_state_uninitialised case too where
> > appopriate.
> > 
> > Finally, make free_heap_pages() call through to init_heap_pages() when
> > given a page range which has not been initialised. This cannot keep
> > recursing because init_heap_pages() will set each page state to
> > PGC_state_inuse before passing it back to free_heap_pages() for the
> > second time.
> > 
> > Signed-off-by: David Woodhouse <[e-mail]>
> > ---
> >  xen/arch/x86/mm.c        |  3 ++-
> >  xen/common/page_alloc.c  | 44 +++++++++++++++++++++++++++++-----------
> >  xen/include/asm-arm/mm.h |  3 ++-
> >  xen/include/asm-x86/mm.h |  3 ++-
> >  4 files changed, 38 insertions(+), 15 deletions(-)
> > 
> > diff --git a/xen/arch/x86/mm.c b/xen/arch/x86/mm.c
> > index 62507ca651..5f0581c072 100644
> > --- a/xen/arch/x86/mm.c
> > +++ b/xen/arch/x86/mm.c
> > @@ -491,7 +491,8 @@ void share_xen_page_with_guest(struct page_info *page, struct domain *d,
> > 
> >      page_set_owner(page, d);
> >      smp_wmb(); /* install valid domain ptr before updating refcnt. */
> > -    ASSERT((page->count_info & ~PGC_xen_heap) == 0);
> > +    ASSERT((page->count_info & ~PGC_xen_heap) == PGC_state_inuse ||
> > +           (page->count_info & ~PGC_xen_heap) == PGC_state_uninitialised);
> 
> Could the page state perhaps be bumped to inuse in this case? It
> seems odd to leave state uninitialized yet succeed in sharing with a
> guest.

No, that doesn't really work.

You can't just *declare* that the page was properly initialised,
because it isn't true. There's a pathological case where the zone
hasn't been initialised at all, because *all* the pages in that zone
got handed out by the boot allocator or used for initrd etc. 

The first pages 'freed' in that zone end up being used (in
init_heap_pages) to create the zone structures.

Likewise, it could include a page which init_heap_pages() doesn't
actually *put* into the buddy allocator, to work around the cross-zone
merge problem. It's fine to use that page and share it with a guest,
but it can't ever be freed into the buddy allocator.

> > @@ -1828,7 +1840,8 @@ static void init_heap_pages(
> >              nr_pages -= n;
> >          }
> > 
> > -        free_heap_pages(pg + i, 0, scrub_debug || idle_scrub);
> 
> Would it be worth an ASSERT(!pg[i].count_info) here in case something
> is added which erroneously modifies the page count info before this
> is done?

That seems valid, I think. Will test it.

> > 
> >          page_set_owner(&pg[i], d);
> >          smp_wmb(); /* Domain pointer must be visible before updating refcnt. */
> > -        pg[i].count_info =
> > -            (pg[i].count_info & PGC_extra) | PGC_allocated | 1;
> > +        pg[i].count_info = (pg[i].count_info & PGC_state) | PGC_allocated | 1;
> 
> The PGC_extra seems to have vapourized here.

Oops. Will fix; thanks.

Paul Durrant March 23, 2020, 8:49 a.m. UTC | #4

> -----Original Message-----
> > > diff --git a/xen/arch/x86/mm.c b/xen/arch/x86/mm.c
> > > index 62507ca651..5f0581c072 100644
> > > --- a/xen/arch/x86/mm.c
> > > +++ b/xen/arch/x86/mm.c
> > > @@ -491,7 +491,8 @@ void share_xen_page_with_guest(struct page_info *page, struct domain *d,
> > >
> > >      page_set_owner(page, d);
> > >      smp_wmb(); /* install valid domain ptr before updating refcnt. */
> > > -    ASSERT((page->count_info & ~PGC_xen_heap) == 0);
> > > +    ASSERT((page->count_info & ~PGC_xen_heap) == PGC_state_inuse ||
> > > +           (page->count_info & ~PGC_xen_heap) == PGC_state_uninitialised);
> >
> > Could the page state perhaps be bumped to inuse in this case? It
> > seems odd to leave state uninitialized yet succeed in sharing with a
> > guest.
> 
> No, that doesn't really work.
> 
> You can't just *declare* that the page was properly initialised,
> because it isn't true. There's a pathological case where the zone
> hasn't been initialised at all, because *all* the pages in that zone
> got handed out by the boot allocator or used for initrd etc.
> 
> The first pages 'freed' in that zone end up being used (in
> init_heap_pages) to create the zone structures.
> 
> Likewise, it could include a page which init_heap_pages() doesn't
> actually *put* into the buddy allocator, to work around the cross-zone
> merge problem. It's fine to use that page and share it with a guest,
> but it can't ever be freed into the buddy allocator.
> 

Ok, so deferring the call to free_heap_pages() (and consequently init_heap_pages()) is safe to defer until the guest is torn down? (Or is this only safe if the page is being assigned to the initial domain?)

  Paul

Julien Grall March 23, 2020, 9:34 a.m. UTC | #5

Hi David,

On 20/03/2020 15:17, David Woodhouse wrote:
> On Fri, 2020-03-20 at 13:33 +0000, Paul Durrant wrote:
>>> -----Original Message-----
>>> From: Xen-devel <[e-mail]> On Behalf Of David Woodhouse
>>> Sent: 19 March 2020 21:22
>>> To: [e-mail]
>>> Cc: Stefano Stabellini <[e-mail]>; Julien Grall <[e-mail]>; Wei Liu <[e-mail]>;
>>> Andrew Cooper <[e-mail]>; Ian Jackson <[e-mail]>; George Dunlap
>>> <[e-mail]>; [e-mail]; Jan Beulich <[e-mail]>; Volodymyr Babchuk
>>> <[e-mail]>; Roger Pau Monné <[e-mail]>
>>> Subject: [Xen-devel] [PATCH 2/2] xen/mm: Introduce PGC_state_uninitialised
>>>
>>> From: David Woodhouse <[e-mail]>
>>>
>>> It is possible for pages to enter general circulation without ever
>>> being process by init_heap_pages().
>>>
>>> For example, pages of the multiboot module containing the initramfs may
>>> be assigned via assign_pages() to dom0 as it is created. And some code
>>> including map_pages_to_xen() has checks on 'system_state' to determine
>>> whether to use the boot or the heap allocator, but it seems impossible
>>> to prove that pages allocated by the boot allocator are not subsequently
>>> freed with free_heap_pages().
>>>
>>> This actually works fine in the majority of cases; there are only a few
>>> esoteric corner cases which init_heap_pages() handles before handing the
>>> page range off to free_heap_pages():
>>>   • Excluding MFN #0 to avoid inappropriate cross-zone merging.
>>>   • Ensuring that the node information structures exist, when the first
>>>     page(s) of a given node are handled.
>>>   • High order allocations crossing from one node to another.
>>>
>>> To handle this case, shift PG_state_inuse from its current value of
>>> zero, to another value. Use zero, which is the initial state of the
>>> entire frame table, as PG_state_uninitialised.
>>>
>>> Fix a couple of assertions which were assuming that PG_state_inuse is
>>> zero, and make them cope with the PG_state_uninitialised case too where
>>> appopriate.
>>>
>>> Finally, make free_heap_pages() call through to init_heap_pages() when
>>> given a page range which has not been initialised. This cannot keep
>>> recursing because init_heap_pages() will set each page state to
>>> PGC_state_inuse before passing it back to free_heap_pages() for the
>>> second time.
>>>
>>> Signed-off-by: David Woodhouse <[e-mail]>
>>> ---
>>>   xen/arch/x86/mm.c        |  3 ++-
>>>   xen/common/page_alloc.c  | 44 +++++++++++++++++++++++++++++-----------
>>>   xen/include/asm-arm/mm.h |  3 ++-
>>>   xen/include/asm-x86/mm.h |  3 ++-
>>>   4 files changed, 38 insertions(+), 15 deletions(-)
>>>
>>> diff --git a/xen/arch/x86/mm.c b/xen/arch/x86/mm.c
>>> index 62507ca651..5f0581c072 100644
>>> --- a/xen/arch/x86/mm.c
>>> +++ b/xen/arch/x86/mm.c
>>> @@ -491,7 +491,8 @@ void share_xen_page_with_guest(struct page_info *page, struct domain *d,
>>>
>>>       page_set_owner(page, d);
>>>       smp_wmb(); /* install valid domain ptr before updating refcnt. */
>>> -    ASSERT((page->count_info & ~PGC_xen_heap) == 0);
>>> +    ASSERT((page->count_info & ~PGC_xen_heap) == PGC_state_inuse ||
>>> +           (page->count_info & ~PGC_xen_heap) == PGC_state_uninitialised);
>>
>> Could the page state perhaps be bumped to inuse in this case? It
>> seems odd to leave state uninitialized yet succeed in sharing with a
>> guest.
> 
> No, that doesn't really work.
> 
> You can't just *declare* that the page was properly initialised,
> because it isn't true. There's a pathological case where the zone
> hasn't been initialised at all, because *all* the pages in that zone
> got handed out by the boot allocator or used for initrd etc.
> 
> The first pages 'freed' in that zone end up being used (in
> init_heap_pages) to create the zone structures.
> 
> Likewise, it could include a page which init_heap_pages() doesn't
> actually *put* into the buddy allocator, to work around the cross-zone
> merge problem. It's fine to use that page and share it with a guest,
> but it can't ever be freed into the buddy allocator.

For liveupdate, we will need a way to initialize a page but mark it as 
already inuse (i.e in the same state as they would be if allocated 
normally).

It feels to me, this is also what we want in this case. The page would 
be first initialize and then we can use it normally including freeing 
later on.

Would it make sense to introduce an helper for this purpose?

Cheers,

David Woodhouse March 23, 2020, 10:45 a.m. UTC | #6

On Mon, 2020-03-23 at 08:49 +0000, Paul Durrant wrote:
> Ok, so deferring the call to free_heap_pages() (and consequently
> init_heap_pages()) is safe to defer until the guest is torn down? (Or
> is this only safe if the page is being assigned to the initial
> domain?)

It's intended to be safe in all cases, including pages which are
allocated from the boot allocator to be used as page tables (cf. the
early-vmap patches), etc.

We kind of have to assume that it's safe to use the page for whatever
purpose it was allocated for, for the lifetime of that usage. If *that*
isn't true, we have bigger problems.

The PGC_state_uninitialised thing is only about recycling it into the
heap *later*, once the lifetime of that initial usage has ended.

David Woodhouse March 23, 2020, 10:55 a.m. UTC | #7

On Mon, 2020-03-23 at 09:34 +0000, Julien Grall wrote:
> For liveupdate, we will need a way to initialize a page but mark it as 
> already inuse (i.e in the same state as they would be if allocated 
> normally).

I am unconvinced of the veracity of this claim.

We don't want to turn specific details of the current Xen buddy
allocator part into of the implicit ABI of live update. That goes for
the power-of-two zone boundaries, amongst other things.

What if Xen receives LU state in which *all* pages in a given zone are
marked as already in use? That's one of the cases in which we *really*
want to pass through init_heap_pages() instead of just
free_heap_pages(), in order to allocate the zone data structures for
the first pages that get freed into that zone.

What if Xen starts to exclude more pages, like the exclusion at zero?

What if new Xen wants to exclude an additional page due to a hardware
erratum? It can't take it away from existing domains (especially if
there are assigned PCI devices) but it could be part of the vetting in
init_heap_pages(), for example.

My intent for PGC_state_uninitialised was to mark pages that haven't
been through init_heap_pages(), whatever init_heap_pages() does in the
current version of Xen.

The pages which are "already in use" because they're inherited through
LU state should be in PGC_state_uninitialised, shouldn't they?

Perhaps if there's a need for a helper, it could be a companion
function to init_heap_pages() which would return a boolean saying,
"nah, I didn't want to do anything to this page anyway", which could
short-circuit it into the PGC_state_inuse state. But I'm not sure I see
the need for such an optimisation.

Julien Grall March 24, 2020, 10:08 a.m. UTC | #8

Hi David,

On 23/03/2020 10:55, David Woodhouse wrote:
> On Mon, 2020-03-23 at 09:34 +0000, Julien Grall wrote:
>> For liveupdate, we will need a way to initialize a page but mark it as
>> already inuse (i.e in the same state as they would be if allocated
>> normally).
> 
> I am unconvinced of the veracity of this claim.
> 
> We don't want to turn specific details of the current Xen buddy
> allocator part into of the implicit ABI of live update. That goes for
> the power-of-two zone boundaries, amongst other things.

Why would you to do that? Marking the page as already used is no 
different to "PGC_state_unitialized" except the "struct page_info" and 
the internal of the buddy allocator would be properly setup for start 
rather than at free.

> 
> What if Xen receives LU state in which *all* pages in a given zone are
> marked as already in use? That's one of the cases in which we *really*
> want to pass through init_heap_pages() instead of just
> free_heap_pages(), in order to allocate the zone data structures for
> the first pages that get freed into that zone.
> 
> What if Xen starts to exclude more pages, like the exclusion at zero?
> 
> What if new Xen wants to exclude an additional page due to a hardware
> erratum? It can't take it away from existing domains (especially if
> there are assigned PCI devices) but it could be part of the vetting in
> init_heap_pages(), for example.

I don't think it would be safe to continue to run a guest using pages 
that were excluded for an HW erratum. It would be safer to not restart 
the domain (or replace the page) in the target Xen if that's hapenning.

> 
> My intent for PGC_state_uninitialised was to mark pages that haven't
> been through init_heap_pages(), whatever init_heap_pages() does in the
> current version of Xen.
> 
> The pages which are "already in use" because they're inherited through
> LU state should be in PGC_state_uninitialised, shouldn't they?

I think using "PGC_state_unitialised" for preserved page is an abuse. I 
understand this is existing in other part of Xen (particularly on x86), 
but I would rather not try to add more.

The PGC_state_unitialised may work for the current allocator because 
most of the fields are not going to be used after allocation. But it may 
not hold for any new allocator (I know the embedded folks are working on 
a new one).

> 
> Perhaps if there's a need for a helper, it could be a companion
> function to init_heap_pages() which would return a boolean saying,
> "nah, I didn't want to do anything to this page anyway", which could
> short-circuit it into the PGC_state_inuse state. But I'm not sure I see
> the need for such an optimisation.

I don't view it as an optimisation but as a way to avoid spreading the 
current misbehavior.

Cheers,

David Woodhouse March 24, 2020, 5:55 p.m. UTC | #9

On Tue, 2020-03-24 at 10:08 +0000, Julien Grall wrote:
> Hi David,
> 
> On 23/03/2020 10:55, David Woodhouse wrote:
> > On Mon, 2020-03-23 at 09:34 +0000, Julien Grall wrote:
> > > For liveupdate, we will need a way to initialize a page but mark it as
> > > already inuse (i.e in the same state as they would be if allocated
> > > normally).
> > 
> > I am unconvinced of the veracity of this claim.
> > 
> > We don't want to turn specific details of the current Xen buddy
> > allocator part into of the implicit ABI of live update. That goes for
> > the power-of-two zone boundaries, amongst other things.
> 
> Why would you to do that? Marking the page as already used is no 
> different to "PGC_state_unitialized" except the "struct page_info" and 
> the internal of the buddy allocator would be properly setup for start 
> rather than at free.

The internals of the buddy allocator *cannot* be set up properly for a
page which it would not actually give out — like the zero page.

We *could* do some tricks to allocate the zone structures for zones
which *do* exist but contain only "pre-allocated" pages so the buddy
allocator has never seen those zones... yet.


> I think using "PGC_state_unitialised" for preserved page is an abuse. I 
> understand this is existing in other part of Xen (particularly on x86), 
> but I would rather not try to add more.


I am perfectly happy to try avoiding PGC_state_uninitialised for pages
which are "in use" at boot time due to live update.

All I insist on is that you explicitly describe the ABI constraints
that it imposes, if any.

For example, that hack which stops the buddy allocator from giving out
page zero: Could we have live updated from a Xen without that hack, to
a Xen which has it? With page zero already given out to a domain?

With PGC_state_initialised and passing the page through
init_heap_pages() if/when it ever gets freed, my answer would be 'yes'.

What's yours? How would we cope with a situation like that, over LU?

Julien Grall March 24, 2020, 6:34 p.m. UTC | #10

Hi David,

On 24/03/2020 17:55, David Woodhouse wrote:
> On Tue, 2020-03-24 at 10:08 +0000, Julien Grall wrote:
>> Hi David,
>>
>> On 23/03/2020 10:55, David Woodhouse wrote:
>>> On Mon, 2020-03-23 at 09:34 +0000, Julien Grall wrote:
>>>> For liveupdate, we will need a way to initialize a page but mark it as
>>>> already inuse (i.e in the same state as they would be if allocated
>>>> normally).
>>>
>>> I am unconvinced of the veracity of this claim.
>>>
>>> We don't want to turn specific details of the current Xen buddy
>>> allocator part into of the implicit ABI of live update. That goes for
>>> the power-of-two zone boundaries, amongst other things.
>>
>> Why would you to do that? Marking the page as already used is no
>> different to "PGC_state_unitialized" except the "struct page_info" and
>> the internal of the buddy allocator would be properly setup for start
>> rather than at free.
> 
> The internals of the buddy allocator *cannot* be set up properly for a
> page which it would not actually give out — like the zero page.
> 
> We *could* do some tricks to allocate the zone structures for zones
> which *do* exist but contain only "pre-allocated" pages so the buddy
> allocator has never seen those zones... yet.
> 
> 
>> I think using "PGC_state_unitialised" for preserved page is an abuse. I
>> understand this is existing in other part of Xen (particularly on x86),
>> but I would rather not try to add more.
> 
> 
> I am perfectly happy to try avoiding PGC_state_uninitialised for pages
> which are "in use" at boot time due to live update.
> 
> All I insist on is that you explicitly describe the ABI constraints
> that it imposes, if any.

Agreed.

> 
> For example, that hack which stops the buddy allocator from giving out
> page zero: Could we have live updated from a Xen without that hack, to
> a Xen which has it? With page zero already given out to a domain?

The buddy allocator could never have given out page 0 on x86 because it 
is part of the first MB of the RAM (see arch_init_memory() in 
arch/x86/mm.c). AFAIK, the first MB cannot be freed..

The change in the buddy allocator was to address the Arm side and also 
make clear this was a problem this is a weakness of the allocator.

> What's yours? How would we cope with a situation like that, over LU?

When do you expect the pages to be carved out from the buddy allocator?

I can see only two situations:
	1) Workaround a bug in the allocator.
         2) A CPU errata requiring to not use memory.

In the case of 1), it is still safe for a domain to run with that page. 
However, we don't want to give it back to the page allocator. A solution 
is to mark them as "offlining/broken". Alternatively, you could remove 
the swap the page (see more below).

In the case of 2), it is not safe for a domain to run with that page. So 
it is probably best to swap the pages with a new one. For HVM domain 
(including the P2M), it should be easy. For PV domain, I am not entirely 
sure if that's feasible.

Cheers,

Jan Beulich March 31, 2020, 12:10 p.m. UTC | #11

On 19.03.2020 22:21, David Woodhouse wrote:
> --- a/xen/arch/x86/mm.c
> +++ b/xen/arch/x86/mm.c
> @@ -491,7 +491,8 @@ void share_xen_page_with_guest(struct page_info *page, struct domain *d,
>  
>      page_set_owner(page, d);
>      smp_wmb(); /* install valid domain ptr before updating refcnt. */
> -    ASSERT((page->count_info & ~PGC_xen_heap) == 0);
> +    ASSERT((page->count_info & ~PGC_xen_heap) == PGC_state_inuse ||
> +           (page->count_info & ~PGC_xen_heap) == PGC_state_uninitialised);

Like for patch 1, there's a risk of the page transitioning from
uninitialised to inuse between these two comparisons, making the
ASSERT() trigger when it shouldn't. As you've got two more
similar constructs further down in the patch, maybe this also
warrants a helper function/macro?

> --- a/xen/common/page_alloc.c
> +++ b/xen/common/page_alloc.c
> @@ -252,6 +252,8 @@ struct bootmem_region {
>  static struct bootmem_region __initdata
>      bootmem_region_list[PAGE_SIZE / sizeof(struct bootmem_region)];
>  static unsigned int __initdata nr_bootmem_regions;
> +static void init_heap_pages(struct page_info *pg, unsigned long nr_pages,
> +                            bool scrub);
>  
>  struct scrub_region {
>      unsigned long offset;
> @@ -1390,6 +1392,17 @@ static void free_heap_pages(
>      ASSERT(order <= MAX_ORDER);
>      ASSERT(node >= 0);
>  
> +    if ( page_state_is(pg, uninitialised) )
> +    {
> +        init_heap_pages(pg, 1 << order, need_scrub);

While, at least for now, it shouldn't matter in practice, and
while code overall is very inconsistent in this regard, with
the respective parameter type being "unsigned long" may I
suggest to use 1UL here?

Jan

Patch

diff --git a/xen/arch/x86/mm.c b/xen/arch/x86/mm.c
index 62507ca651..5f0581c072 100644
--- a/xen/arch/x86/mm.c
+++ b/xen/arch/x86/mm.c
@@ -491,7 +491,8 @@  void share_xen_page_with_guest(struct page_info *page, struct domain *d,
 
     page_set_owner(page, d);
     smp_wmb(); /* install valid domain ptr before updating refcnt. */
-    ASSERT((page->count_info & ~PGC_xen_heap) == 0);
+    ASSERT((page->count_info & ~PGC_xen_heap) == PGC_state_inuse ||
+           (page->count_info & ~PGC_xen_heap) == PGC_state_uninitialised);
 
     /* Only add to the allocation list if the domain isn't dying. */
     if ( !d->is_dying )
diff --git a/xen/common/page_alloc.c b/xen/common/page_alloc.c
index 8d72a64f4e..4f7971f2a1 100644
--- a/xen/common/page_alloc.c
+++ b/xen/common/page_alloc.c
@@ -252,6 +252,8 @@  struct bootmem_region {
 static struct bootmem_region __initdata
     bootmem_region_list[PAGE_SIZE / sizeof(struct bootmem_region)];
 static unsigned int __initdata nr_bootmem_regions;
+static void init_heap_pages(struct page_info *pg, unsigned long nr_pages,
+                            bool scrub);
 
 struct scrub_region {
     unsigned long offset;
@@ -1390,6 +1392,17 @@  static void free_heap_pages(
     ASSERT(order <= MAX_ORDER);
     ASSERT(node >= 0);
 
+    if ( page_state_is(pg, uninitialised) )
+    {
+        init_heap_pages(pg, 1 << order, need_scrub);
+        /*
+         * init_heap_pages() will call back into free_heap_pages() for
+         * each page but cannot keep recursing because each page will
+         * be set to PGC_state_inuse first.
+         */
+        return;
+    }
+
     spin_lock(&heap_lock);
 
     for ( i = 0; i < (1 << order); i++ )
@@ -1771,11 +1784,10 @@  int query_page_offline(mfn_t mfn, uint32_t *status)
  * latter is not on a MAX_ORDER boundary, then we reserve the page by
  * not freeing it to the buddy allocator.
  */
-static void init_heap_pages(
-    struct page_info *pg, unsigned long nr_pages)
+static void init_heap_pages(struct page_info *pg, unsigned long nr_pages,
+                            bool scrub)
 {
     unsigned long i;
-    bool idle_scrub = false;
 
     /*
      * Keep MFN 0 away from the buddy allocator to avoid crossing zone
@@ -1800,7 +1812,7 @@  static void init_heap_pages(
     spin_unlock(&heap_lock);
 
     if ( system_state < SYS_STATE_active && opt_bootscrub == BOOTSCRUB_IDLE )
-        idle_scrub = true;
+        scrub = true;
 
     for ( i = 0; i < nr_pages; i++ )
     {
@@ -1828,7 +1840,8 @@  static void init_heap_pages(
             nr_pages -= n;
         }
 
-        free_heap_pages(pg + i, 0, scrub_debug || idle_scrub);
+        pg[i].count_info = PGC_state_inuse;
+        free_heap_pages(pg + i, 0, scrub_debug || scrub);
     }
 }
 
@@ -1864,7 +1877,7 @@  void __init end_boot_allocator(void)
         if ( (r->s < r->e) &&
              (phys_to_nid(pfn_to_paddr(r->s)) == cpu_to_node(0)) )
         {
-            init_heap_pages(mfn_to_page(_mfn(r->s)), r->e - r->s);
+            init_heap_pages(mfn_to_page(_mfn(r->s)), r->e - r->s, false);
             r->e = r->s;
             break;
         }
@@ -1873,7 +1886,7 @@  void __init end_boot_allocator(void)
     {
         struct bootmem_region *r = &bootmem_region_list[i];
         if ( r->s < r->e )
-            init_heap_pages(mfn_to_page(_mfn(r->s)), r->e - r->s);
+            init_heap_pages(mfn_to_page(_mfn(r->s)), r->e - r->s, false);
     }
     nr_bootmem_regions = 0;
 
@@ -2142,7 +2155,7 @@  void init_xenheap_pages(paddr_t ps, paddr_t pe)
 
     memguard_guard_range(maddr_to_virt(ps), pe - ps);
 
-    init_heap_pages(maddr_to_page(ps), (pe - ps) >> PAGE_SHIFT);
+    init_heap_pages(maddr_to_page(ps), (pe - ps) >> PAGE_SHIFT, false);
 }
 
 
@@ -2251,7 +2264,7 @@  void init_domheap_pages(paddr_t ps, paddr_t pe)
     if ( mfn_x(emfn) <= mfn_x(smfn) )
         return;
 
-    init_heap_pages(mfn_to_page(smfn), mfn_x(emfn) - mfn_x(smfn));
+    init_heap_pages(mfn_to_page(smfn), mfn_x(emfn) - mfn_x(smfn), false);
 }
 
 
@@ -2280,7 +2293,8 @@  int assign_pages(
 
         for ( i = 0; i < (1ul << order); i++ )
         {
-            ASSERT(!(pg[i].count_info & ~PGC_extra));
+            ASSERT((pg[i].count_info & ~PGC_extra) == PGC_state_inuse ||
+                   (pg[i].count_info & ~PGC_extra) == PGC_state_uninitialised);
             if ( pg[i].count_info & PGC_extra )
                 extra_pages++;
         }
@@ -2316,10 +2330,16 @@  int assign_pages(
     for ( i = 0; i < (1 << order); i++ )
     {
         ASSERT(page_get_owner(&pg[i]) == NULL);
+        /*
+         * Note: Not using page_state_is() here. The ASSERT requires that
+         * all other bits in count_info are zero, in addition to PGC_state
+         * being appropriate.
+         */
+        ASSERT((pg[i].count_info & ~PGC_extra) == PGC_state_inuse ||
+               (pg[i].count_info & ~PGC_extra) == PGC_state_uninitialised);
         page_set_owner(&pg[i], d);
         smp_wmb(); /* Domain pointer must be visible before updating refcnt. */
-        pg[i].count_info =
-            (pg[i].count_info & PGC_extra) | PGC_allocated | 1;
+        pg[i].count_info = (pg[i].count_info & PGC_state) | PGC_allocated | 1;
         page_list_add_tail(&pg[i], &d->page_list);
     }
 
diff --git a/xen/include/asm-arm/mm.h b/xen/include/asm-arm/mm.h
index a877791d1c..49663fa98a 100644
--- a/xen/include/asm-arm/mm.h
+++ b/xen/include/asm-arm/mm.h
@@ -113,12 +113,13 @@  struct page_info
   * { inuse, offlining, offlined, free, broken_offlining, broken }
   */
 #define PGC_state                  PG_mask(7, 9)
-#define PGC_state_inuse            PG_mask(0, 9)
+#define PGC_state_uninitialised    PG_mask(0, 9)
 #define PGC_state_offlining        PG_mask(1, 9)
 #define PGC_state_offlined         PG_mask(2, 9)
 #define PGC_state_free             PG_mask(3, 9)
 #define PGC_state_broken_offlining PG_mask(4, 9) /* Broken and offlining */
 #define PGC_state_broken           PG_mask(5, 9) /* Broken and offlined */
+#define PGC_state_inuse            PG_mask(6, 9)
 
 #define pgc_is(pgc, st)            (((pgc) & PGC_state) == PGC_state_##st)
 #define page_state_is(pg, st)       pgc_is((pg)->count_info, st)
diff --git a/xen/include/asm-x86/mm.h b/xen/include/asm-x86/mm.h
index 1203f1b179..5fbbca5f05 100644
--- a/xen/include/asm-x86/mm.h
+++ b/xen/include/asm-x86/mm.h
@@ -72,12 +72,13 @@ 
   * { inuse, offlining, offlined, free, broken_offlining, broken }
   */
 #define PGC_state                  PG_mask(7, 9)
-#define PGC_state_inuse            PG_mask(0, 9)
+#define PGC_state_uninitialised    PG_mask(0, 9)
 #define PGC_state_offlining        PG_mask(1, 9)
 #define PGC_state_offlined         PG_mask(2, 9)
 #define PGC_state_free             PG_mask(3, 9)
 #define PGC_state_broken_offlining PG_mask(4, 9) /* Broken and offlining */
 #define PGC_state_broken           PG_mask(5, 9) /* Broken and offlined */
+#define PGC_state_inuse            PG_mask(6, 9)
 
 #define pgc_is(pgc, st)            (((pgc) & PGC_state) == PGC_state_##st)
 #define page_state_is(pg, st)       pgc_is((pg)->count_info, st)