[v3,0/8] mm/mprotect: Fix dax puds

Message

Peter Xu July 15, 2024, 7:21 p.m. UTC

[Based on mm-unstable, commit 31334cf98dbd, July 2nd]

v3:
- Fix a build issue on i386 PAE config
- Moved one line from patch 8 to patch 3

v1: https://lore.kernel.org/r/[e-mail]
v2: https://lore.kernel.org/r/[e-mail]

Dax supports pud pages for a while, but mprotect on puds was missing since
the start.  This series tries to fix that by providing pud handling in
mprotect().  The goal is to add more types of pud mappings like hugetlb or
pfnmaps.  This series paves way for it by fixing known pud entries.

Considering nobody reported this until when I looked at those other types
of pud mappings, I am thinking maybe it doesn't need to be a fix for stable
and this may not need to be backported.  I would guess whoever cares about
mprotect() won't care 1G dax puds yet, vice versa.  I hope fixing that in
new kernels would be fine, but I'm open to suggestions.

There're a few small things changed to teach mprotect work on PUDs. E.g. it
will need to start with dropping NUMA_HUGE_PTE_UPDATES which may stop
making sense when there can be more than one type of huge pte.  OTOH, we'll
also need to push the mmu notifiers from pmd to pud layers, which might
need some attention but so far I think it's safe.  For such details, please
refer to each patch's commit message.

The mprotect() pud process should be straightforward, as I kept it as
simple as possible.  There's no NUMA handled as dax simply doesn't support
that.  There's also no userfault involvements as file memory (even if work
with userfault-wp async mode) will need to split a pud, so pud entry
doesn't need to yet know userfault's existance (but hugetlb entries will;
that's also for later).

Tests
=====

What I did test:

- cross-build tests that I normally cover [1]

- smoke tested on x86_64 the simplest program [2] on dev_dax 1G PUD
  mprotect() using QEMU's nvdimm emulations [3] and ndctl to create
  namespaces with proper alignments, which used to throw "bad pud" but now
  it'll run through all fine.  I checked sigbus happens if with illegal
  access on protected puds.

- vmtests.

What I didn't test:

- fsdax: I wanted to also give it a shot, but only until then I noticed it
  doesn't seem to be supported (according to dax_iomap_fault(), which will
  always fallback on PUD_ORDER).  I did remember it was supported before, I
  could miss something important there.. please shoot if so.

- userfault wp-async: I also wanted to test userfault-wp async be able to
  split huge puds (here it's simply a clear_pud.. though), but it won't
  work for devdax anyway due to not allowed to do smaller than 1G faults in
  this case. So skip too.

- Power, as no hardware on hand.

Thanks,

[1] https://gitlab.com/peterx/lkb-harness/-/blob/main/config.json
[2] https://github.com/xzpeter/clibs/blob/master/misc/dax.c
[3] https://github.com/qemu/qemu/blob/master/docs/nvdimm.txt

Peter Xu (8):
  mm/dax: Dump start address in fault handler
  mm/mprotect: Remove NUMA_HUGE_PTE_UPDATES
  mm/mprotect: Push mmu notifier to PUDs
  mm/powerpc: Add missing pud helpers
  mm/x86: Make pud_leaf() only cares about PSE bit
  mm/x86: arch_check_zapped_pud()
  mm/x86: Add missing pud helpers
  mm/mprotect: fix dax pud handlings

 arch/powerpc/include/asm/book3s/64/pgtable.h |  3 +
 arch/powerpc/mm/book3s64/pgtable.c           | 20 ++++++
 arch/x86/include/asm/pgtable.h               | 68 +++++++++++++++---
 arch/x86/mm/pgtable.c                        | 19 +++++
 drivers/dax/device.c                         |  6 +-
 include/linux/huge_mm.h                      | 24 +++++++
 include/linux/pgtable.h                      |  7 ++
 include/linux/vm_event_item.h                |  1 -
 mm/huge_memory.c                             | 56 ++++++++++++++-
 mm/mprotect.c                                | 74 ++++++++++++--------
 mm/vmstat.c                                  |  1 -
 11 files changed, 234 insertions(+), 45 deletions(-)

Comments

Peter Xu July 15, 2024, 8 p.m. UTC | #1

On Mon, Jul 15, 2024 at 03:21:34PM -0400, Peter Xu wrote:
> [Based on mm-unstable, commit 31334cf98dbd, July 2nd]

I forgot to update here in the cover letter; it's actually based on the
lastest..  Which is 79ae458094ff, as of today (July 15th).

Peter Xu July 24, 2024, 3:15 p.m. UTC | #2

On Mon, Jul 15, 2024 at 03:21:34PM -0400, Peter Xu wrote:
> [Based on mm-unstable, commit 31334cf98dbd, July 2nd]
> 
> v3:
> - Fix a build issue on i386 PAE config
> - Moved one line from patch 8 to patch 3
> 
> v1: https://lore.kernel.org/r/[e-mail]
> v2: https://lore.kernel.org/r/[e-mail]
> 
> Dax supports pud pages for a while, but mprotect on puds was missing since
> the start.  This series tries to fix that by providing pud handling in
> mprotect().  The goal is to add more types of pud mappings like hugetlb or
> pfnmaps.  This series paves way for it by fixing known pud entries.
> 
> Considering nobody reported this until when I looked at those other types
> of pud mappings, I am thinking maybe it doesn't need to be a fix for stable
> and this may not need to be backported.  I would guess whoever cares about
> mprotect() won't care 1G dax puds yet, vice versa.  I hope fixing that in
> new kernels would be fine, but I'm open to suggestions.
> 
> There're a few small things changed to teach mprotect work on PUDs. E.g. it
> will need to start with dropping NUMA_HUGE_PTE_UPDATES which may stop
> making sense when there can be more than one type of huge pte.  OTOH, we'll
> also need to push the mmu notifiers from pmd to pud layers, which might
> need some attention but so far I think it's safe.  For such details, please
> refer to each patch's commit message.
> 
> The mprotect() pud process should be straightforward, as I kept it as
> simple as possible.  There's no NUMA handled as dax simply doesn't support
> that.  There's also no userfault involvements as file memory (even if work
> with userfault-wp async mode) will need to split a pud, so pud entry
> doesn't need to yet know userfault's existance (but hugetlb entries will;
> that's also for later).
> 
> Tests
> =====
> 
> What I did test:
> 
> - cross-build tests that I normally cover [1]
> 
> - smoke tested on x86_64 the simplest program [2] on dev_dax 1G PUD
>   mprotect() using QEMU's nvdimm emulations [3] and ndctl to create
>   namespaces with proper alignments, which used to throw "bad pud" but now
>   it'll run through all fine.  I checked sigbus happens if with illegal
>   access on protected puds.
> 
> - vmtests.
> 
> What I didn't test:
> 
> - fsdax: I wanted to also give it a shot, but only until then I noticed it
>   doesn't seem to be supported (according to dax_iomap_fault(), which will
>   always fallback on PUD_ORDER).  I did remember it was supported before, I
>   could miss something important there.. please shoot if so.
> 
> - userfault wp-async: I also wanted to test userfault-wp async be able to
>   split huge puds (here it's simply a clear_pud.. though), but it won't
>   work for devdax anyway due to not allowed to do smaller than 1G faults in
>   this case. So skip too.
> 
> - Power, as no hardware on hand.

Ping - any review comments or even tests would be greatly welcomed.

I'm not sure whether this matters for anyone yet so far.  I hope this still
makes sense for DAX even if this is an extremely corner case...

Just to mention the follow up users of this path:

  - huge pfnmap 1G may use this, when VM_PFNMAP can be mapped with 1G too,
    then we should hit similar "bad pud" here.

  - hugetlb rework will use this, when we want this path to process 1G
    hugetlb pages too.

The 1st user is not a must in my initial plan, as VFIO + VM use case
doesn't use mprotect(), so we can keep (1) broken together with DAX 1G
here.  But for the long term we should still fix this, IMHO.

Thanks,