xfs: clear _XBF_PAGES from buffers when readahead page

djwong · djwong · commit 2aa6ba7b5ad3 · 2017-01-25T20:24:57.000-08:00
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 &lt;darrick.wong@oracle.com&gt;
Reviewed-by: Eric Sandeen &lt;sandeen@redhat.com&gt;
diff --git a/fs/xfs/xfs_buf.c b/fs/xfs/xfs_buf.c
@@ -422,6 +422,7 @@ xfs_buf_allocate_memory(
 out_free_pages:
 	for (i = 0; i < bp->b_page_count; i++)
 		__free_page(bp->b_pages[i]);
+	bp->b_flags &= ~_XBF_PAGES;
 	return error;
 }
 

Original file line number	Diff line number	Diff line change
`@@ -422,6 +422,7 @@ xfs_buf_allocate_memory(`
`422`	`422`	`out_free_pages:`
`423`	`423`	`for (i = 0; i < bp->b_page_count; i++)`
`424`	`424`	`__free_page(bp->b_pages[i]);`
	`425`	`+ bp->b_flags &= ~_XBF_PAGES;`
`425`	`426`	`return error;`
`426`	`427`	`}`
`427`	`428`