[v2,2/3] xen/privcmd: Mark pages as dirty
diff mbox series

Message ID 1594059372-15563-3-git-send-email-jrdr.linux@gmail.com
State Superseded
Headers show
Series
  • Few bug fixes and Convert to pin_user_pages*()
Related show

Commit Message

Souptick Joarder July 6, 2020, 6:16 p.m. UTC
pages need to be marked as dirty before unpinned it in
unlock_pages() which was oversight. This is fixed now.

Signed-off-by: Souptick Joarder <jrdr.linux@gmail.com>
Suggested-by: John Hubbard <jhubbard@nvidia.com>
Cc: John Hubbard <jhubbard@nvidia.com>
Cc: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Cc: Paul Durrant <xadimgnik@gmail.com>
---
 drivers/xen/privcmd.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

Comments

Jürgen Groß July 7, 2020, 9:38 a.m. UTC | #1
On 06.07.20 20:16, Souptick Joarder wrote:
> pages need to be marked as dirty before unpinned it in
> unlock_pages() which was oversight. This is fixed now.
> 
> Signed-off-by: Souptick Joarder <jrdr.linux@gmail.com>
> Suggested-by: John Hubbard <jhubbard@nvidia.com>
> Cc: John Hubbard <jhubbard@nvidia.com>
> Cc: Boris Ostrovsky <boris.ostrovsky@oracle.com>
> Cc: Paul Durrant <xadimgnik@gmail.com>
> ---
>   drivers/xen/privcmd.c | 5 ++++-
>   1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/xen/privcmd.c b/drivers/xen/privcmd.c
> index 33677ea..f6c1543 100644
> --- a/drivers/xen/privcmd.c
> +++ b/drivers/xen/privcmd.c
> @@ -612,8 +612,11 @@ static void unlock_pages(struct page *pages[], unsigned int nr_pages)
>   {
>   	unsigned int i;
>   
> -	for (i = 0; i < nr_pages; i++)
> +	for (i = 0; i < nr_pages; i++) {
> +		if (!PageDirty(pages[i]))
> +			set_page_dirty_lock(pages[i]);

With put_page() directly following I think you should be able to use
set_page_dirty() instead, as there is obviously a reference to the page
existing.

>   		put_page(pages[i]);
> +	}
>   }
>   
>   static long privcmd_ioctl_dm_op(struct file *file, void __user *udata)
> 

Juergen
Souptick Joarder July 7, 2020, 11:30 a.m. UTC | #2
On Tue, Jul 7, 2020 at 3:08 PM Jürgen Groß <jgross@suse.com> wrote:
>
> On 06.07.20 20:16, Souptick Joarder wrote:
> > pages need to be marked as dirty before unpinned it in
> > unlock_pages() which was oversight. This is fixed now.
> >
> > Signed-off-by: Souptick Joarder <jrdr.linux@gmail.com>
> > Suggested-by: John Hubbard <jhubbard@nvidia.com>
> > Cc: John Hubbard <jhubbard@nvidia.com>
> > Cc: Boris Ostrovsky <boris.ostrovsky@oracle.com>
> > Cc: Paul Durrant <xadimgnik@gmail.com>
> > ---
> >   drivers/xen/privcmd.c | 5 ++++-
> >   1 file changed, 4 insertions(+), 1 deletion(-)
> >
> > diff --git a/drivers/xen/privcmd.c b/drivers/xen/privcmd.c
> > index 33677ea..f6c1543 100644
> > --- a/drivers/xen/privcmd.c
> > +++ b/drivers/xen/privcmd.c
> > @@ -612,8 +612,11 @@ static void unlock_pages(struct page *pages[], unsigned int nr_pages)
> >   {
> >       unsigned int i;
> >
> > -     for (i = 0; i < nr_pages; i++)
> > +     for (i = 0; i < nr_pages; i++) {
> > +             if (!PageDirty(pages[i]))
> > +                     set_page_dirty_lock(pages[i]);
>
> With put_page() directly following I think you should be able to use
> set_page_dirty() instead, as there is obviously a reference to the page
> existing.

Patch [3/3] will convert above codes to use unpin_user_pages_dirty_lock()
which internally do the same check. So I thought to keep linux-stable and
linux-next code in sync. John had a similar concern [1] and later agreed to keep
this check.

Shall I keep this check ?  No ?

[1] https://lore.kernel.org/xen-devel/a750e5e5-fd5d-663b-c5fd-261d7c939ba7@nvidia.com/

>
> >               put_page(pages[i]);
> > +     }
> >   }
> >
> >   static long privcmd_ioctl_dm_op(struct file *file, void __user *udata)
> >
>
> Juergen
Jürgen Groß July 7, 2020, 11:43 a.m. UTC | #3
On 07.07.20 13:30, Souptick Joarder wrote:
> On Tue, Jul 7, 2020 at 3:08 PM Jürgen Groß <jgross@suse.com> wrote:
>>
>> On 06.07.20 20:16, Souptick Joarder wrote:
>>> pages need to be marked as dirty before unpinned it in
>>> unlock_pages() which was oversight. This is fixed now.
>>>
>>> Signed-off-by: Souptick Joarder <jrdr.linux@gmail.com>
>>> Suggested-by: John Hubbard <jhubbard@nvidia.com>
>>> Cc: John Hubbard <jhubbard@nvidia.com>
>>> Cc: Boris Ostrovsky <boris.ostrovsky@oracle.com>
>>> Cc: Paul Durrant <xadimgnik@gmail.com>
>>> ---
>>>    drivers/xen/privcmd.c | 5 ++++-
>>>    1 file changed, 4 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/drivers/xen/privcmd.c b/drivers/xen/privcmd.c
>>> index 33677ea..f6c1543 100644
>>> --- a/drivers/xen/privcmd.c
>>> +++ b/drivers/xen/privcmd.c
>>> @@ -612,8 +612,11 @@ static void unlock_pages(struct page *pages[], unsigned int nr_pages)
>>>    {
>>>        unsigned int i;
>>>
>>> -     for (i = 0; i < nr_pages; i++)
>>> +     for (i = 0; i < nr_pages; i++) {
>>> +             if (!PageDirty(pages[i]))
>>> +                     set_page_dirty_lock(pages[i]);
>>
>> With put_page() directly following I think you should be able to use
>> set_page_dirty() instead, as there is obviously a reference to the page
>> existing.
> 
> Patch [3/3] will convert above codes to use unpin_user_pages_dirty_lock()
> which internally do the same check. So I thought to keep linux-stable and
> linux-next code in sync. John had a similar concern [1] and later agreed to keep
> this check.
> 
> Shall I keep this check ?  No ?
> 
> [1] https://lore.kernel.org/xen-devel/a750e5e5-fd5d-663b-c5fd-261d7c939ba7@nvidia.com/

I wasn't referring to checking PageDirty(), but to the use of
set_page_dirty_lock().

Looking at the comment just before the implementation of
set_page_dirty_lock() suggests that it is fine to use set_page_dirty()
instead (so not calling lock_page()).

Only the transition from get_user_pages_fast() to pin_user_pages_fast()
requires to use the locked version IMO.


Juergen
John Hubbard July 7, 2020, 7:30 p.m. UTC | #4
On 2020-07-07 04:43, Jürgen Groß wrote:
> On 07.07.20 13:30, Souptick Joarder wrote:
>> On Tue, Jul 7, 2020 at 3:08 PM Jürgen Groß <jgross@suse.com> wrote:
...
>>>> diff --git a/drivers/xen/privcmd.c b/drivers/xen/privcmd.c
>>>> index 33677ea..f6c1543 100644
>>>> --- a/drivers/xen/privcmd.c
>>>> +++ b/drivers/xen/privcmd.c
>>>> @@ -612,8 +612,11 @@ static void unlock_pages(struct page *pages[], unsigned int nr_pages)
>>>>    {
>>>>        unsigned int i;
>>>>
>>>> -     for (i = 0; i < nr_pages; i++)
>>>> +     for (i = 0; i < nr_pages; i++) {
>>>> +             if (!PageDirty(pages[i]))
>>>> +                     set_page_dirty_lock(pages[i]);
>>>
>>> With put_page() directly following I think you should be able to use
>>> set_page_dirty() instead, as there is obviously a reference to the page
>>> existing.
>>
>> Patch [3/3] will convert above codes to use unpin_user_pages_dirty_lock()
>> which internally do the same check. So I thought to keep linux-stable and
>> linux-next code in sync. John had a similar concern [1] and later agreed to keep
>> this check.
>>
>> Shall I keep this check ?  No ?

It doesn't matter *too* much, because patch 3/3 fixes up everything by
changing it all to unpin_user_pages_dirty_lock(). However, there is something
to be said for having correct interim patches, too. :)  Details:

>>
>> [1] https://lore.kernel.org/xen-devel/a750e5e5-fd5d-663b-c5fd-261d7c939ba7@nvidia.com/
> 
> I wasn't referring to checking PageDirty(), but to the use of
> set_page_dirty_lock().
> 
> Looking at the comment just before the implementation of
> set_page_dirty_lock() suggests that it is fine to use set_page_dirty()
> instead (so not calling lock_page()).


no no, that's a misreading of the comment. Unless this xen/privcmd code has
somehow taken a reference on page->mapping->host (which I do *not* think is
the case), then it is still racy to call set_page_dirty() here. Instead,
set_page_dirty_lock() should be used.


> 
> Only the transition from get_user_pages_fast() to pin_user_pages_fast()
> requires to use the locked version IMO.
> 

That's a different misunderstanding. :) pin_user_pages*() APIs are meant to be
functionally drop-in replacements for get_user_pages*(). Internally,
pin_user_pages*() functions do some additional tracking, but from a caller's
perspective, it should look the same. In other words, there is nothing
about pin_user_pages_fast() that requires set_page_dirty_lock() upon release.
The reason set_page_dirty_lock() was chosen is that there are very few
(none at all?) call sites that need to release and dirty a page, that also meet
the requirements to safely call set_page_dirty().

That's why there is a unpin_user_pages_dirty_lock(), but there is not a
corresponding unpin_user_pages_dirty() call: the latter has not been required
so far, even though the call site conversions are nearly done.


thanks,
Jürgen Groß July 8, 2020, 5:40 a.m. UTC | #5
On 07.07.20 21:30, John Hubbard wrote:
> On 2020-07-07 04:43, Jürgen Groß wrote:
>> On 07.07.20 13:30, Souptick Joarder wrote:
>>> On Tue, Jul 7, 2020 at 3:08 PM Jürgen Groß <jgross@suse.com> wrote:
> ...
>>>>> diff --git a/drivers/xen/privcmd.c b/drivers/xen/privcmd.c
>>>>> index 33677ea..f6c1543 100644
>>>>> --- a/drivers/xen/privcmd.c
>>>>> +++ b/drivers/xen/privcmd.c
>>>>> @@ -612,8 +612,11 @@ static void unlock_pages(struct page *pages[], 
>>>>> unsigned int nr_pages)
>>>>>    {
>>>>>        unsigned int i;
>>>>>
>>>>> -     for (i = 0; i < nr_pages; i++)
>>>>> +     for (i = 0; i < nr_pages; i++) {
>>>>> +             if (!PageDirty(pages[i]))
>>>>> +                     set_page_dirty_lock(pages[i]);
>>>>
>>>> With put_page() directly following I think you should be able to use
>>>> set_page_dirty() instead, as there is obviously a reference to the page
>>>> existing.
>>>
>>> Patch [3/3] will convert above codes to use 
>>> unpin_user_pages_dirty_lock()
>>> which internally do the same check. So I thought to keep linux-stable 
>>> and
>>> linux-next code in sync. John had a similar concern [1] and later 
>>> agreed to keep
>>> this check.
>>>
>>> Shall I keep this check ?  No ?
> 
> It doesn't matter *too* much, because patch 3/3 fixes up everything by
> changing it all to unpin_user_pages_dirty_lock(). However, there is 
> something
> to be said for having correct interim patches, too. :)  Details:
> 
>>>
>>> [1] 
>>> https://lore.kernel.org/xen-devel/a750e5e5-fd5d-663b-c5fd-261d7c939ba7@nvidia.com/ 
>>>
>>
>> I wasn't referring to checking PageDirty(), but to the use of
>> set_page_dirty_lock().
>>
>> Looking at the comment just before the implementation of
>> set_page_dirty_lock() suggests that it is fine to use set_page_dirty()
>> instead (so not calling lock_page()).
> 
> 
> no no, that's a misreading of the comment. Unless this xen/privcmd code has
> somehow taken a reference on page->mapping->host (which I do *not* think is
> the case), then it is still racy to call set_page_dirty() here. Instead,
> set_page_dirty_lock() should be used.

Ah, okay. Thanks for the clarification.

So you can add my

Reviewed-by: Juergen Gross <jgross@suse.com>


Juergen

Patch
diff mbox series

diff --git a/drivers/xen/privcmd.c b/drivers/xen/privcmd.c
index 33677ea..f6c1543 100644
--- a/drivers/xen/privcmd.c
+++ b/drivers/xen/privcmd.c
@@ -612,8 +612,11 @@  static void unlock_pages(struct page *pages[], unsigned int nr_pages)
 {
 	unsigned int i;
 
-	for (i = 0; i < nr_pages; i++)
+	for (i = 0; i < nr_pages; i++) {
+		if (!PageDirty(pages[i]))
+			set_page_dirty_lock(pages[i]);
 		put_page(pages[i]);
+	}
 }
 
 static long privcmd_ioctl_dm_op(struct file *file, void __user *udata)