[Cluster-devel] [PATCH] gfs2: Fix loop in gfs2_rbm_find (2)

[Andreas Gruenbacher <agruenba@redhat.com>]

The fix from commit 2d29f6b96d8f introduced an unexpected performance
regression when allocating blocks.  Fix by rewriting the overly
complicated wrap-around logic in gfs2_rbm_find in a more reasonable way.
Discovered and verified with iozone.

Fixes: 2d29f6b96d8f ("gfs2: Fix loop in gfs2_rbm_find")
Cc: stable at vger.kernel.org # v3.13+
Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>
---
 fs/gfs2/rgrp.c | 40 +++++++++++++---------------------------
 1 file changed, 13 insertions(+), 27 deletions(-)

diff --git a/fs/gfs2/rgrp.c b/fs/gfs2/rgrp.c
index 831d7cb5a49c4..3f16411d885d9 100644
--- a/fs/gfs2/rgrp.c
+++ b/fs/gfs2/rgrp.c
@@ -1730,25 +1730,15 @@ static int gfs2_rbm_find(struct gfs2_rbm *rbm, u8 state, u32 *minext,
 			 const struct gfs2_inode *ip, bool nowrap)
 {
 	struct buffer_head *bh;
-	int initial_bii;
-	u32 initial_offset;
 	int first_bii = rbm->bii;
 	u32 first_offset = rbm->offset;
 	u32 offset;
 	u8 *buffer;
-	int n = 0;
-	int iters = rbm->rgd->rd_length;
+	bool wrapped = false;
 	int ret;
 	struct gfs2_bitmap *bi;
 	struct gfs2_extent maxext = { .rbm.rgd = rbm->rgd, };
 
-	/* If we are not starting at the beginning of a bitmap, then we
-	 * need to add one to the bitmap count to ensure that we search
-	 * the starting bitmap twice.
-	 */
-	if (rbm->offset != 0)
-		iters++;
-
 	while(1) {
 		bi = rbm_bi(rbm);
 		if ((ip == NULL || !gfs2_rs_active(&ip->i_res)) &&
@@ -1761,47 +1751,43 @@ static int gfs2_rbm_find(struct gfs2_rbm *rbm, u8 state, u32 *minext,
 		WARN_ON(!buffer_uptodate(bh));
 		if (state != GFS2_BLKST_UNLINKED && bi->bi_clone)
 			buffer = bi->bi_clone + bi->bi_offset;
-		initial_offset = rbm->offset;
 		offset = gfs2_bitfit(buffer, bi->bi_bytes, rbm->offset, state);
-		if (offset == BFITNOENT)
-			goto bitmap_full;
+		if (offset == BFITNOENT) {
+			if (state == GFS2_BLKST_FREE && rbm->offset == 0)
+				set_bit(GBF_FULL, &bi->bi_flags);
+			goto next_bitmap;
+		}
 		rbm->offset = offset;
 		if (ip == NULL)
 			return 0;
 
-		initial_bii = rbm->bii;
 		ret = gfs2_reservation_check_and_update(rbm, ip,
 							minext ? *minext : 0,
 							&maxext);
 		if (ret == 0)
 			return 0;
-		if (ret > 0) {
-			n += (rbm->bii - initial_bii);
+		if (ret > 0)
 			goto next_iter;
-		}
 		if (ret == -E2BIG) {
-			n += rbm->bii - initial_bii;
 			rbm->bii = 0;
 			rbm->offset = 0;
 			goto res_covered_end_of_rgrp;
 		}
 		return ret;
 
-bitmap_full:	/* Mark bitmap as full and fall through */
-		if ((state == GFS2_BLKST_FREE) && initial_offset == 0)
-			set_bit(GBF_FULL, &bi->bi_flags);
-
 next_bitmap:	/* Find next bitmap in the rgrp */
 		rbm->offset = 0;
 		rbm->bii++;
 		if (rbm->bii == rbm->rgd->rd_length)
 			rbm->bii = 0;
 res_covered_end_of_rgrp:
-		if ((rbm->bii == 0) && nowrap)
-			break;
-		n++;
+		if (rbm->bii == 0) {
+			if (nowrap)
+				break;
+			wrapped = true;
+		}
 next_iter:
-		if (n >= iters)
+		if (wrapped && rbm->bii > first_bii)
 			break;
 	}
 
-- 
2.20.1