Patch "xfs: Timely free truncated dirty pages" has been added to the 4.9-stable tree

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

xfs: Timely free truncated dirty pages

to the 4.9-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-timely-free-truncated-dirty-pages.patch
and it can be found in the queue-4.9 subdirectory.

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

From: Jan Kara <[email protected]>
Date: Wed, 11 Jan 2017 10:20:04 -0800
Subject: xfs: Timely free truncated dirty pages
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From: Jan Kara <[email protected]>

commit 0a417b8dc1f10b03e8f558b8a831f07ec4c23795 upstream.

Commit 99579ccec4e2 "xfs: skip dirty pages in ->releasepage()" started to skip dirty pages in xfs_vm_releasepage() which also has the effect that if a dirty page is truncated, it does not get freed by block_invalidatepage() and is lingering in LRU list waiting for reclaim. So a simple loop like:

while true; do

	dd if=/dev/zero of=file bs=1M count=100
	rm file

done

will keep using more and more memory until we hit low watermarks and start pagecache reclaim which will eventually reclaim also the truncate pages. Keeping these truncated (and thus never usable) pages in memory is just a waste of memory, is unnecessarily stressing page cache reclaim, and reportedly also leads to anonymous mmap(2) returning ENOMEM prematurely.

So instead of just skipping dirty pages in xfs_vm_releasepage(), return to old behavior of skipping them only if they have delalloc or unwritten buffers and fix the spurious warnings by warning only if the page is clean.

---
 fs/xfs/xfs_aops.c |   19 +++++++++++--------
 1 file changed, 11 insertions(+), 8 deletions(-)

@@ -1158,19 +1158,22 @@ xfs_vm_releasepage(
 	 * block_invalidatepage() can send pages that are still marked dirty
 	 * but otherwise have invalidated buffers.
 	 *
-	 * We've historically freed buffers on the latter. Instead, quietly
-	 * filter out all dirty pages to avoid spurious buffer state warnings.
-	 * This can likely be removed once shrink_active_list() is fixed.
+	 * We want to release the latter to avoid unnecessary buildup of the
+	 * LRU, skip the former and warn if we've left any lingering
+	 * delalloc/unwritten buffers on clean pages. Skip pages with delalloc
+	 * or unwritten buffers and warn if the page is not dirty. Otherwise
+	 * try to release the buffers.
 	 */
-	if (PageDirty(page))
-		return 0;
-
 	xfs_count_page_state(page, &delalloc, &unwritten);
 
-	if (WARN_ON_ONCE(delalloc))
+	if (delalloc) {
+		WARN_ON_ONCE(!PageDirty(page));
 		return 0;
-	if (WARN_ON_ONCE(unwritten))
+	}
+	if (unwritten) {
+		WARN_ON_ONCE(!PageDirty(page));
 		return 0;
+	}
 
 	return try_to_free_buffers(page);
 }

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

queue-4.9/xfs-timely-free-truncated-dirty-pages.patch queue-4.9/dax-fix-deadlock-with-dax-4k-holes.patch