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

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Message-Id: <1497897167-14556-64-git-send-email-w@1wt.eu>
Date:   Mon, 19 Jun 2017 20:29:22 +0200
From:   Willy Tarreau <w@....eu>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org,
        linux@...ck-us.net
Cc:     "Darrick J. Wong" <darrick.wong@...cle.com>,
        Ivan Kozik <ivan@...ios.org>, Jiri Slaby <jslaby@...e.cz>,
        Willy Tarreau <w@....eu>
Subject: [PATCH 3.10 063/268] xfs: clear _XBF_PAGES from buffers when readahead page

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>
Cc: Ivan Kozik <ivan@...ios.org>
Signed-off-by: Jiri Slaby <jslaby@...e.cz>
Signed-off-by: Willy Tarreau <w@....eu>
---
 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:
 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;
 }

-- 
2.8.0.rc2.1.gbe9624a