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

From 2aa6ba7b5ad3189cc27f14540aa2f57f0ed8df4b Mon Sep 17 00:00:00 2001

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

Quoted 1 line

From 2aa6ba7b5ad3189cc27f14540aa2f57f0ed8df4b Mon Sep 17 00:00:00 2001

From: "Darrick J. Wong" <[email protected]>
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 <[email protected]>

Reviewed-by Eric Sandeen <[email protected]>

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

fs/xfs/xfs_buf.c +1 -0

index 7f0a01f7b592..ac3b4db519df 100644
@@ -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;
 }
 

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2.11.0

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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.