Patch "xfs: clear _XBF_PAGES from buffers when readahead page" has been added to the 4.4-stable tree

This is a note to let you know that I've just added the patch titled

xfs: clear _XBF_PAGES from buffers when readahead page

to the 4.4-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
xfs-clear-_xbf_pages-from-buffers-when-readahead-page.patch
and it can be found in the queue-4.4 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: "Darrick J. Wong" <[email protected]>
Date: Wed, 25 Jan 2017 20:24:57 -0800
Subject: xfs: clear _XBF_PAGES from buffers when readahead page

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

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.

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

@@ -375,6 +375,7 @@ 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;
 }
 

Patches currently in stable-queue which might be from [email protected] are

queue-4.4/xfs-don-t-allow-di_size-with-high-bit-set.patch queue-4.4/xfs-clear-_xbf_pages-from-buffers-when-readahead-page.patch