Commit 7fcbbaf1 authored May 22, 2014 by Jens Axboe Committed by Linus Torvalds May 23, 2014

mm/filemap.c: avoid always dirtying mapping->flags on O_DIRECT



In some testing I ran today (some fio jobs that spread over two nodes),
we end up spending 40% of the time in filemap_check_errors().  That
smells fishy.  Looking further, this is basically what happens:

blkdev_aio_read()
    generic_file_aio_read()
        filemap_write_and_wait_range()
            if (!mapping->nr_pages)
                filemap_check_errors()

and filemap_check_errors() always attempts two test_and_clear_bit() on
the mapping flags, thus dirtying it for every single invocation.  The
patch below tests each of these bits before clearing them, avoiding this
issue.  In my test case (4-socket box), performance went from 1.7M IOPS
to 4.0M IOPS.

Signed-off-by: Jens Axboe <axboe@fb.com>
Acked-by: Jeff Moyer <jmoyer@redhat.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

parent b985194c

mm/filemap.c

+4 −2

Original line number	Diff line number	Diff line
		@@ -257,9 +257,11 @@ static int filemap_check_errors(struct address_space *mapping)
		{
		int ret = 0;
		/* Check for outstanding write errors */
		if (test_and_clear_bit(AS_ENOSPC, &mapping->flags))
		if (test_bit(AS_ENOSPC, &mapping->flags) &&
		test_and_clear_bit(AS_ENOSPC, &mapping->flags))
		ret = -ENOSPC;
		if (test_and_clear_bit(AS_EIO, &mapping->flags))
		if (test_bit(AS_EIO, &mapping->flags) &&
		test_and_clear_bit(AS_EIO, &mapping->flags))
		ret = -EIO;
		return ret;
		}

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