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>

4 files changed, 11 insertions, 6 deletions

diff --git a/fs/gfs2/bmap.c b/fs/gfs2/bmap.c
index 9fa3aef9a5b3..fa3ea29f39cf 100644
--- a/fs/gfs2/bmap.c
+++ b/fs/gfs2/bmap.c
@@ -291,8 +291,9 @@ static void gfs2_metapath_ra(struct gfs2_glock *gl,
                 if (trylock_buffer(rabh)) {
                         if (!buffer_uptodate(rabh)) {
                                 rabh->b_end_io = end_buffer_read_sync;
-                                submit_bh(REQ_OP_READ, REQ_RAHEAD | REQ_META,
-                                                rabh);
+                                submit_bh(REQ_OP_READ,
+                                          REQ_RAHEAD | REQ_META | REQ_PRIO,
+                                          rabh);
                                 continue;
                         }
                         unlock_buffer(rabh);
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);
diff --git a/fs/gfs2/meta_io.c b/fs/gfs2/meta_io.c
index 4da7745c890a..61ef6c9be816 100644
--- a/fs/gfs2/meta_io.c
+++ b/fs/gfs2/meta_io.c
@@ -453,7 +453,7 @@ struct buffer_head *gfs2_meta_ra(struct gfs2_glock *gl, u64 dblock, u32 extlen)
         if (buffer_uptodate(first_bh))
                 goto out;
         if (!buffer_locked(first_bh))
-                ll_rw_block(REQ_OP_READ, REQ_META, 1, &first_bh);
+                ll_rw_block(REQ_OP_READ, REQ_META | REQ_PRIO, 1, &first_bh);
 
         dblock++;
         extlen--;
@@ -462,7 +462,9 @@ struct buffer_head *gfs2_meta_ra(struct gfs2_glock *gl, u64 dblock, u32 extlen)
                 bh = gfs2_getbuf(gl, dblock, CREATE);
 
                 if (!buffer_uptodate(bh) && !buffer_locked(bh))
-                        ll_rw_block(REQ_OP_READ, REQ_RAHEAD | REQ_META, 1, &bh);
+                        ll_rw_block(REQ_OP_READ,
+                                    REQ_RAHEAD | REQ_META | REQ_PRIO,
+                                    1, &bh);
                 brelse(bh);
                 dblock++;
                 extlen--;
diff --git a/fs/gfs2/quota.c b/fs/gfs2/quota.c
index c2ca9566b764..739adf105d7f 100644
--- a/fs/gfs2/quota.c
+++ b/fs/gfs2/quota.c
@@ -730,7 +730,7 @@ static int gfs2_write_buf_to_page(struct gfs2_inode *ip, unsigned long index,
                 if (PageUptodate(page))
                         set_buffer_uptodate(bh);
                 if (!buffer_uptodate(bh)) {
-                        ll_rw_block(REQ_OP_READ, REQ_META, 1, &bh);
+                        ll_rw_block(REQ_OP_READ, REQ_META | REQ_PRIO, 1, &bh);
                         wait_on_buffer(bh);
                         if (!buffer_uptodate(bh))
                                 goto unlock_out;