linux-kernel - Re: [PATCH BUGFIX] mm: fix find_get_page() for shmem exceptional entries

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Date:	Tue, 7 Feb 2012 10:31:36 -0800 (PST)
From:	Hugh Dickins <hughd@...gle.com>
To:	Konstantin Khlebnikov <khlebnikov@...nvz.org>
cc:	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH BUGFIX] mm: fix find_get_page() for shmem exceptional
 entries

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@...nvz.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))
--
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