gfs2: add flag REQ_PRIO for metadata I/O

When gfs2 does metadata I/O, only REQ_META is used as a metadata hint of
the bio. But flag REQ_META is just a hint for block trace, not for block
layer code to handle a bio as metadata request.

For some of metadata I/Os of gfs2, A REQ_PRIO flag on the metadata bio
would be very informative to block layer code. For example, if bcache is
used as a I/O cache for gfs2, it will be possible for bcache code to get
the hint and cache the pre-fetched metadata blocks on cache device. This
behavior may be helpful to improve metadata I/O performance if the
following requests hit the cache.

Here are the locations in gfs2 code where a REQ_PRIO flag should be added,
- All places where REQ_READAHEAD is used, gfs2 code uses this flag for
  metadata read ahead.
- In gfs2_meta_rq() where the first metadata block is read in.
- In gfs2_write_buf_to_page(), read in quota metadata blocks to have them
  up to date.
These metadata blocks are probably to be accessed again in future, adding
a REQ_PRIO flag may have bcache to keep such metadata in fast cache
device. For system without a cache layer, REQ_PRIO can still provide hint
to block layer to handle metadata requests more properly.

Signed-off-by: Coly Li <colyli@suse.de>
Signed-off-by: Bob Peterson <rpeterso@redhat.com>

1 files changed, 3 insertions, 1 deletions

diff --git a/fs/gfs2/dir.c b/fs/gfs2/dir.c
index db427658ccd9..0741e4018f8c 100644
--- a/fs/gfs2/dir.c
+++ b/fs/gfs2/dir.c
@@ -1514,7 +1514,9 @@ static void gfs2_dir_readahead(struct inode *inode, unsigned hsize, u32 index,
 				continue;
 			}
 			bh->b_end_io = end_buffer_read_sync;
-			submit_bh(REQ_OP_READ, REQ_RAHEAD | REQ_META, bh);
+			submit_bh(REQ_OP_READ,
+				  REQ_RAHEAD | REQ_META | REQ_PRIO,
+				  bh);
 			continue;
 		}
 		brelse(bh);