linux-kernel - [PATCH 4.4 42/43] mm/filemap.c: dont initiate writeback if mapping has no dirty pages

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Date:   Mon, 11 Nov 2019 19:28:56 +0100
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org,
        Konstantin Khlebnikov <khlebnikov@...dex-team.ru>,
        Jan Kara <jack@...e.cz>, Tejun Heo <tj@...nel.org>,
        Jens Axboe <axboe@...nel.dk>,
        Johannes Weiner <hannes@...xchg.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: [PATCH 4.4 42/43] mm/filemap.c: dont initiate writeback if mapping has no dirty pages

From: Konstantin Khlebnikov <khlebnikov@...dex-team.ru>

commit c3aab9a0bd91b696a852169479b7db1ece6cbf8c upstream.

Functions like filemap_write_and_wait_range() should do nothing if inode
has no dirty pages or pages currently under writeback.  But they anyway
construct struct writeback_control and this does some atomic operations if
CONFIG_CGROUP_WRITEBACK=y - on fast path it locks inode->i_lock and
updates state of writeback ownership, on slow path might be more work.
Current this path is safely avoided only when inode mapping has no pages.

For example generic_file_read_iter() calls filemap_write_and_wait_range()
at each O_DIRECT read - pretty hot path.

This patch skips starting new writeback if mapping has no dirty tags set.
If writeback is already in progress filemap_write_and_wait_range() will
wait for it.

Link: http://lkml.kernel.org/r/156378816804.1087.8607636317907921438.stgit@buzz
Signed-off-by: Konstantin Khlebnikov <khlebnikov@...dex-team.ru>
Reviewed-by: Jan Kara <jack@...e.cz>
Cc: Tejun Heo <tj@...nel.org>
Cc: Jens Axboe <axboe@...nel.dk>
Cc: Johannes Weiner <hannes@...xchg.org>
Signed-off-by: Andrew Morton <akpm@...ux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@...ux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>

---
 mm/filemap.c |    3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -340,7 +340,8 @@ int __filemap_fdatawrite_range(struct ad
 		.range_end = end,
 	};

-	if (!mapping_cap_writeback_dirty(mapping))
+	if (!mapping_cap_writeback_dirty(mapping) ||
+	    !mapping_tagged(mapping, PAGECACHE_TAG_DIRTY))
 		return 0;

 	wbc_attach_fdatawrite_inode(&wbc, mapping->host);