mm: fix find_get_page() for shmem exceptional entries

It should return NULL, otherwise the caller will be very surprised.

Signed-off-by Konstantin Khlebnikov<[email protected]>

---
  mm/filemap.c |    1 +
  1 files changed, 1 insertions(+), 0 deletions(-)

mm/filemap.c +0 -0

index 518223b..ca98cb5 100644

Bug was added in commit v3.0-7291-g8079b1c (mm: clarify the radix_tree exceptional cases)
So, v3.1 and v3.2 affected.

Quoted 3 lines · Konstantin Khlebnikov

It should return NULL, otherwise the caller will be very surprised.

Signed-off-by Konstantin Khlebnikov<[email protected]>

Quoted 4 lines

---
  mm/filemap.c |    1 +
  1 files changed, 1 insertions(+), 0 deletions(-)

mm/filemap.c repeat: · line 699

Quoted 12 lines

mm/filemap.c +1 -0

index 518223b..ca98cb5 100644
@@ -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))

Quoted 7 lines · Konstantin Khlebnikov

Bug was added in commit v3.0-7291-g8079b1c (mm: clarify the radix_tree exceptional cases)
So, v3.1 and v3.2 affected.

Quoted 2 lines · Konstantin Khlebnikov

It should return NULL, otherwise the caller will be very surprised.

Quoted 1 line

Quoted 1 line

Signed-off-by Konstantin Khlebnikov<[email protected]>

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

Quoted 4 lines

Quoted 4 lines

---
   mm/filemap.c |    1 +
   1 files changed, 1 insertions(+), 0 deletions(-)

Quoted 4 lines

Quoted 4 lines

mm/filemap.c +0 -0

index 518223b..ca98cb5 100644

Quoted 8 lines

mm/filemap.c repeat: · line 699

Quoted 8 lines

mm/filemap.c +1 -0

@@ -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))