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>

1 files changed, 4 insertions, 2 deletions

diff --git a/mm/filemap.c b/mm/filemap.c
index 000a220e2a41..088358c8006b 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -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;
 }