Request 2aa6ba7b5ad3 ("clear _XBF_PAGES from buffers when readahead page") for 4.4 stable inclusion

[Ivan Kozik <ivan@ludios.org>]

Hi,

I would like to request that this patch be included in the 4.4 stable tree.  It 
fixes the Bad page state issue discovered at 
http://oss.sgi.com/archives/xfs/2016-08/msg00617.html ('"Bad page state" errors 
when calling BULKSTAT under memory pressure?')

I tested the patch (no changes needed) by applying it to 4.4.52, running a 
program to use almost all of my free memory, then running xfs_fsr on a 
filesystem with > 1.5M files.  Before patch: kernel screams with Bad page state 
/ "count:-1" within a minute.  After patch: no complaints from the kernel. 
I repeated the test several times and on another machine that was affected. 
I have not seen any problems five days later.

Thanks,

Ivan

>From 2aa6ba7b5ad3189cc27f14540aa2f57f0ed8df4b Mon Sep 17 00:00:00 2001
From: "Darrick J. Wong" <darrick.wong@oracle.com>
Date: Wed, 25 Jan 2017 20:24:57 -0800
Subject: [PATCH] xfs: clear _XBF_PAGES from buffers when readahead page

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@oracle.com>
Reviewed-by: Eric Sandeen <sandeen@redhat.com>
---
 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 7f0a01f7b592..ac3b4db519df 100644
--- a/fs/xfs/xfs_buf.c
+++ b/fs/xfs/xfs_buf.c
@@ -422,6 +422,7 @@ 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;
 }
 
-- 
2.11.0