linux-kernel - [PATCH 3.18 104/124] xfs: clear _XBF_PAGES from buffers when readahead page

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Message-Id: <20170420063601.057258124@linuxfoundation.org>
Date:   Thu, 20 Apr 2017 08:36:19 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org,
        "Darrick J. Wong" <darrick.wong@...cle.com>,
        Eric Sandeen <sandeen@...hat.com>
Subject: [PATCH 3.18 104/124] xfs: clear _XBF_PAGES from buffers when readahead page

3.18-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Darrick J. Wong <darrick.wong@...cle.com>

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@...cle.com>
Reviewed-by: Eric Sandeen <sandeen@...hat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>

---
 fs/xfs/xfs_buf.c |    1 +
 1 file changed, 1 insertion(+)

--- a/fs/xfs/xfs_buf.c
+++ b/fs/xfs/xfs_buf.c
@@ -376,6 +376,7 @@ retry:
 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;
 }