[PATCH] xfs: Timely free truncated dirty pages

[Jan Kara <jack@suse.cz>]

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 is also confusing users thinking they are running out of
memory.

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.

CC: Brian Foster <bfoster@redhat.com>
CC: Vlastimil Babka <vbabka@suse.cz>
Reported-by: Petr Tůma <petr.tuma@d3s.mff.cuni.cz>
Fixes: 99579ccec4e271c3d4d4e7c946058766812afdab
Signed-off-by: Jan Kara <jack@suse.cz>
---
 fs/xfs/xfs_aops.c | 19 +++++++++----------
 1 file changed, 9 insertions(+), 10 deletions(-)

diff --git a/fs/xfs/xfs_aops.c b/fs/xfs/xfs_aops.c
index 0f56fcd3a5d5..670d38ff7dc7 100644
--- a/fs/xfs/xfs_aops.c
+++ b/fs/xfs/xfs_aops.c
@@ -1150,21 +1150,20 @@ xfs_vm_releasepage(
 	 * the dirty bit cleared. Thus, it can send actual dirty pages to
 	 * ->releasepage() via shrink_active_list(). Conversely,
 	 * 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.
+	 * but otherwise have invalidated buffers. So we warn only if the page
+	 * is clean to avoid spurious warnings when called from
+	 * shrink_active_list() for a dirty page.
 	 */
-	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);
 }
-- 
2.10.2