* [PATCH BUGFIX] mm: fix find_get_page() for shmem exceptional entries
@ 2012-02-07 10:31 Konstantin Khlebnikov
2012-02-07 10:43 ` Konstantin Khlebnikov
0 siblings, 1 reply; 4+ messages in thread
From: Konstantin Khlebnikov @ 2012-02-07 10:31 UTC (permalink / raw)
To: linux-mm, Andrew Morton, Hugh Dickins, linux-kernel
It should return NULL, otherwise the caller will be very surprised.
Signed-off-by: Konstantin Khlebnikov <khlebnikov@openvz.org>
---
mm/filemap.c | 1 +
1 files changed, 1 insertions(+), 0 deletions(-)
diff --git a/mm/filemap.c b/mm/filemap.c
index 518223b..ca98cb5 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -693,6 +693,7 @@ repeat:
* here as an exceptional entry: so return it without
* attempting to raise page count.
*/
+ page = NULL;
goto out;
}
if (!page_cache_get_speculative(page))
^ permalink raw reply related [flat|nested] 4+ messages in thread
* Re: [PATCH BUGFIX] mm: fix find_get_page() for shmem exceptional entries
2012-02-07 10:31 [PATCH BUGFIX] mm: fix find_get_page() for shmem exceptional entries Konstantin Khlebnikov
@ 2012-02-07 10:43 ` Konstantin Khlebnikov
2012-02-07 18:31 ` Hugh Dickins
0 siblings, 1 reply; 4+ messages in thread
From: Konstantin Khlebnikov @ 2012-02-07 10:43 UTC (permalink / raw)
To: linux-mm, Andrew Morton, Hugh Dickins, linux-kernel
Bug was added in commit v3.0-7291-g8079b1c (mm: clarify the radix_tree exceptional cases)
So, v3.1 and v3.2 affected.
Konstantin Khlebnikov wrote:
> It should return NULL, otherwise the caller will be very surprised.
>
> Signed-off-by: Konstantin Khlebnikov<khlebnikov@openvz.org>
> ---
> mm/filemap.c | 1 +
> 1 files changed, 1 insertions(+), 0 deletions(-)
>
> diff --git a/mm/filemap.c b/mm/filemap.c
> index 518223b..ca98cb5 100644
> --- a/mm/filemap.c
> +++ b/mm/filemap.c
> @@ -693,6 +693,7 @@ repeat:
> * here as an exceptional entry: so return it without
> * attempting to raise page count.
> */
> + page = NULL;
> goto out;
> }
> if (!page_cache_get_speculative(page))
>
^ permalink raw reply [flat|nested] 4+ messages in thread
* Re: [PATCH BUGFIX] mm: fix find_get_page() for shmem exceptional entries
2012-02-07 10:43 ` Konstantin Khlebnikov
@ 2012-02-07 18:31 ` Hugh Dickins
2012-02-08 1:41 ` Konstantin Khlebnikov
0 siblings, 1 reply; 4+ messages in thread
From: Hugh Dickins @ 2012-02-07 18:31 UTC (permalink / raw)
To: Konstantin Khlebnikov; +Cc: linux-mm, Andrew Morton, linux-kernel
On Tue, 7 Feb 2012, Konstantin Khlebnikov wrote:
> Bug was added in commit v3.0-7291-g8079b1c (mm: clarify the radix_tree
> exceptional cases)
> So, v3.1 and v3.2 affected.
>
> Konstantin Khlebnikov wrote:
> > It should return NULL, otherwise the caller will be very surprised.
> >
> > Signed-off-by: Konstantin Khlebnikov<khlebnikov@openvz.org>
Thanks for worrying about it, but Nak to this patch.
If you have found somewhere that is surprised by an exceptional entry
instead of a page, then indeed we shall need to fix that: I'm not
aware of any.
There are several places that are prepared for the possibility:
find_lock_page() (and your patch would be breaking shmem.c's use of
find_lock_page()), mincore_page(), memcontrol.c's mc_handle_file_pte().
Of the remaining calls to find_get_page(), my understanding is that
either they are filesystems operating upon their own pagecache, or
they involve using ->readpage() - that's one of the two reasons why
I gave shmem its own ->splice_read() and removed its ->readpage()
before switching over to use the exceptional entries.
Hugh
> > ---
> > mm/filemap.c | 1 +
> > 1 files changed, 1 insertions(+), 0 deletions(-)
> >
> > diff --git a/mm/filemap.c b/mm/filemap.c
> > index 518223b..ca98cb5 100644
> > --- a/mm/filemap.c
> > +++ b/mm/filemap.c
> > @@ -693,6 +693,7 @@ repeat:
> > * here as an exceptional entry: so return it without
> > * attempting to raise page count.
> > */
> > + page = NULL;
> > goto out;
> > }
> > if (!page_cache_get_speculative(page))
^ permalink raw reply [flat|nested] 4+ messages in thread
* Re: [PATCH BUGFIX] mm: fix find_get_page() for shmem exceptional entries
2012-02-07 18:31 ` Hugh Dickins
@ 2012-02-08 1:41 ` Konstantin Khlebnikov
0 siblings, 0 replies; 4+ messages in thread
From: Konstantin Khlebnikov @ 2012-02-08 1:41 UTC (permalink / raw)
To: Hugh Dickins; +Cc: linux-mm, Andrew Morton, linux-kernel
Hugh Dickins wrote:
> On Tue, 7 Feb 2012, Konstantin Khlebnikov wrote:
>
>> Bug was added in commit v3.0-7291-g8079b1c (mm: clarify the radix_tree
>> exceptional cases)
>> So, v3.1 and v3.2 affected.
>>
>> Konstantin Khlebnikov wrote:
>>> It should return NULL, otherwise the caller will be very surprised.
>>>
>>> Signed-off-by: Konstantin Khlebnikov<khlebnikov@openvz.org>
>
> Thanks for worrying about it, but Nak to this patch.
>
> If you have found somewhere that is surprised by an exceptional entry
> instead of a page, then indeed we shall need to fix that: I'm not
> aware of any.
Oh, this is very dangerous semantics, especially for function called "find-get-page"
which sometimes returns not-getted not-a-page =)
>
> There are several places that are prepared for the possibility:
> find_lock_page() (and your patch would be breaking shmem.c's use of
> find_lock_page()), mincore_page(), memcontrol.c's mc_handle_file_pte().
>
> Of the remaining calls to find_get_page(), my understanding is that
> either they are filesystems operating upon their own pagecache, or
> they involve using ->readpage() - that's one of the two reasons why
> I gave shmem its own ->splice_read() and removed its ->readpage()
> before switching over to use the exceptional entries.
>
> Hugh
>
>>> ---
>>> mm/filemap.c | 1 +
>>> 1 files changed, 1 insertions(+), 0 deletions(-)
>>>
>>> diff --git a/mm/filemap.c b/mm/filemap.c
>>> index 518223b..ca98cb5 100644
>>> --- a/mm/filemap.c
>>> +++ b/mm/filemap.c
>>> @@ -693,6 +693,7 @@ repeat:
>>> * here as an exceptional entry: so return it without
>>> * attempting to raise page count.
>>> */
>>> + page = NULL;
>>> goto out;
>>> }
>>> if (!page_cache_get_speculative(page))
^ permalink raw reply [flat|nested] 4+ messages in thread
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2012-02-07 10:31 [PATCH BUGFIX] mm: fix find_get_page() for shmem exceptional entries Konstantin Khlebnikov
2012-02-07 10:43 ` Konstantin Khlebnikov
2012-02-07 18:31 ` Hugh Dickins
2012-02-08 1:41 ` Konstantin Khlebnikov
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