[PATCH 3.10 063/268] xfs: clear _XBF_PAGES from buffers when readahead page

From: Willy Tarreau
Date: Mon Jun 19 2017 - 14:34:47 EST

From: "Darrick J. Wong" <darrick.wong@xxxxxxxxxx>

commit 2aa6ba7b5ad3189cc27f14540aa2f57f0ed8df4b upstream.

If we try to allocate memory pages to back an xfs_buf that we're trying
to read, it's possible that we'll be so short on memory that the page
allocation fails. For a blocking read we'll just wait, but for
readahead we simply dump all the pages we've collected so far.

Unfortunately, after dumping the pages we neglect to clear the
_XBF_PAGES state, which means that the subsequent call to xfs_buf_free
thinks that b_pages still points to pages we own. It then double-frees
the b_pages pages.

This results in screaming about negative page refcounts from the memory
manager, which xfs oughtn't be triggering. To reproduce this case,
mount a filesystem where the size of the inodes far outweighs the
availalble memory (a ~500M inode filesystem on a VM with 300MB memory
did the trick here) and run bulkstat in parallel with other memory
eating processes to put a huge load on the system. The "check summary"
phase of xfs_scrub also works for this purpose.

Signed-off-by: Darrick J. Wong <darrick.wong@xxxxxxxxxx>
Reviewed-by: Eric Sandeen <sandeen@xxxxxxxxxx>
Cc: Ivan Kozik <ivan@xxxxxxxxxx>
Signed-off-by: Jiri Slaby <jslaby@xxxxxxx>
Signed-off-by: Willy Tarreau <w@xxxxxx>
fs/xfs/xfs_buf.c | 1 +
1 file changed, 1 insertion(+)

diff --git a/fs/xfs/xfs_buf.c b/fs/xfs/xfs_buf.c
index 1b2472a..8ff89db 100644
--- a/fs/xfs/xfs_buf.c
+++ b/fs/xfs/xfs_buf.c
@@ -428,6 +428,7 @@ retry:
for (i = 0; i < bp->b_page_count; i++)
+ bp->b_flags &= ~_XBF_PAGES;
return error;