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Commit c3aab9a0 authored by Konstantin Khlebnikov's avatar Konstantin Khlebnikov Committed by Linus Torvalds
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mm/filemap.c: don't initiate writeback if mapping has no dirty pages 

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: default avatarKonstantin Khlebnikov <khlebnikov@yandex-team.ru>
Reviewed-by: default avatarJan Kara <jack@suse.cz>
Cc: Tejun Heo <tj@kernel.org>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent 8974558f

mm/filemap.c

+2 −1
Original line number Diff line number Diff line
@@ -408,7 +408,8 @@ int __filemap_fdatawrite_range(struct address_space *mapping, loff_t start, 
		.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);