[Cluster-devel] [GFS2 PATCH] GFS2: Write the log descriptor entries in the correct order

[Bob Peterson <rpeterso@redhat.com>]

Hi,

I recently discovered GFS2 was writing the log descriptor items in
the journal in reverse order. For example, if I do a bunch of file
creates, the result was a log header that looked something like this:

0x298 (j+ 242): Log descriptor, type 300 (Metadata) len:504, data1: 503
       0xa6e9ce di        0xa6e9a7 di        0xa6e97b di        0xa6e942 di
       0xa6e911 di        0xa6e8d2 di        0xa6e898 di        0xa6e86e di
       0xa6e840 di        0xa6e809 di        0xa6e7d7 di        0xa6e77e di
       0xa6e747 di        0xa6e721 di        0xa6e6f8 di        0xa6e6ca di
       0xa6e680 di        0xa6e63d di        0xa6e619 di        0xa6e5f4 di
       0xa6e5d1 di        0xa6e598 di        0xa6e520 di        0xa6e4f4 di
       0xa6e4c1 di        0xa6e489 di        0xa6e45c di        0xa6e427 di

As you can see, the blocks are reverse-sorted. This patch changes the
order so that they're written in sorted order. Log headers written with
the patch look more like this:

0x36e (j+ 318): Log descriptor, type 300 (Metadata) len:504, data1: 503
       0x4d5a80 di        0x4d5aa0 lf        0x4d5aa1 di        0x4d5aa2 di
       0x4d5aa3 di        0x4d5aa4 di        0x4d5aa5 di        0x4d5aa6 di
       0x4d5aa7 di        0x4d5aa8 di        0x4d5aa9 di        0x4d5aaa di
       0x4d5aab di        0x4d5aac di        0x4d5aad di        0x4d5aae di
       0x4d5aaf di        0x4d5ab0 di        0x4d5ab1 di        0x4d5ab2 di
       0x4d5ab3 di        0x4d5ab4 di        0x4d5ab5 di        0x4d5ab6 di
       0x4d5ab7 di        0x4d5ab8 di        0x4d5ab9 di        0x4d5aba di

The blocks following the log descriptor are written sequentially, so the
sort order won't affect performance at all, but it makes the journal
easier to decipher for debugging purposes.

Patch description:

This patch changes the order in which the blocks are processed by function
gfs2_before_commit. Before, it was processing them in the wrong order such
that the blocks would be layed out and written in reverse order. This
changes the order to the correct order.

Regards,

Bob Peterson
Red Hat File Systems

Signed-off-by: Bob Peterson <rpeterso@redhat.com> 
---
diff --git a/fs/gfs2/lops.c b/fs/gfs2/lops.c
index 2c1ae86..d644470 100644
--- a/fs/gfs2/lops.c
+++ b/fs/gfs2/lops.c
@@ -444,7 +444,7 @@ static void gfs2_before_commit(struct gfs2_sbd *sdp, unsigned int limit,
 		ptr = (__be64 *)(ld + 1);
 
 		n = 0;
-		list_for_each_entry_continue(bd1, blist, bd_list) {
+		list_for_each_entry_continue_reverse(bd1, blist, bd_list) {
 			*ptr++ = cpu_to_be64(bd1->bd_bh->b_blocknr);
 			if (is_databuf) {
 				gfs2_check_magic(bd1->bd_bh);
@@ -459,7 +459,7 @@ static void gfs2_before_commit(struct gfs2_sbd *sdp, unsigned int limit,
 		gfs2_log_lock(sdp);
 
 		n = 0;
-		list_for_each_entry_continue(bd2, blist, bd_list) {
+		list_for_each_entry_continue_reverse(bd2, blist, bd_list) {
 			get_bh(bd2->bd_bh);
 			gfs2_log_unlock(sdp);
 			lock_buffer(bd2->bd_bh);