Re: [PATCH BUGFIX] mm: fix find_get_page() for shmem exceptionalentries

From: Konstantin Khlebnikov
Date: Tue Feb 07 2012 - 20:41:54 EST

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@xxxxxxxxxx>

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.


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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