From c3aab9a0bd91b696a852169479b7db1ece6cbf8c Mon Sep 17 00:00:00 2001 From: Konstantin Khlebnikov Date: Mon, 23 Sep 2019 15:34:45 -0700 Subject: [PATCH] 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: Konstantin Khlebnikov Reviewed-by: Jan Kara Cc: Tejun Heo Cc: Jens Axboe Cc: Johannes Weiner Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- mm/filemap.c | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-) diff --git a/mm/filemap.c b/mm/filemap.c index 5f30aedd7363..82f3b9a3a940 100644 --- a/mm/filemap.c +++ b/mm/filemap.c @@ -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);