[PATCH] xfs_repair: refuse to unset lazycount on V5 filesystems

[Eric Sandeen <sandeen@redhat.com>]

Running "xfs_admin -c 0" on a V5 fs has unfortunate results.

Before:
versionnum [0xbca5+0x18a] = ... LAZYSBCOUNT,PROJID32BIT,CRC, ...
After:
versionnum [0xbca5+0x188] = ... LAZYSBCOUNT,PROJID32BIT,CRC, ...

The on-disk feature bit is in fact changed, but xfs_db reports that the
feature is still present.  This is because xfs_sb_version_haslazysbcount()
ignores the feature bit, and always reports that the lazy-count feature is
present on a V5 fs.

i.e. the intent is that this feature is not optional on a V5 fs, yet we
still flip the bit on disk when asked.  Not good.

So, document this, and error out if the user tries to disable lazy-count
on a version 5 filesystem.

Reported-by: Richard W.M. Jones <rjones@redhat.com>
Signed-off-by: Eric Sandeen <sandeen@redhat.com>
---

(hm, xfs_repair should probably also turn the bit back on, if it's off on
V5?  I'll save that for another patch.)

diff --git a/man/man8/xfs_admin.8 b/man/man8/xfs_admin.8
index b393d74..0968196 100644
--- a/man/man8/xfs_admin.8
+++ b/man/man8/xfs_admin.8
@@ -62,6 +62,10 @@ Print the current filesystem UUID (Universally Unique IDentifier).
 .TP
 .BR "\-c 0" | 1
 Enable (1) or disable (0) lazy-counters in the filesystem.
+.IP
+Lazy-counters may not be disabled on Version 5 superblock filesystems
+(i.e. those with metadata CRCs enabled).
+.IP
 This operation may take quite a bit of time on large filesystems as the
 entire filesystem needs to be scanned when this option is changed.
 .IP

diff --git a/repair/phase1.c b/repair/phase1.c
index ec75ada..9e7011d 100644
--- a/repair/phase1.c
+++ b/repair/phase1.c
@@ -121,8 +121,12 @@ phase1(xfs_mount_t *mp)
 			sb->sb_bad_features2 |= XFS_SB_VERSION2_LAZYSBCOUNTBIT;
 			primary_sb_modified = 1;
 			printf(_("Enabling lazy-counters\n"));
-		} else
-		if (!lazy_count && xfs_sb_version_haslazysbcount(sb)) {
+		} else if (!lazy_count && xfs_sb_version_haslazysbcount(sb)) {
+			if (XFS_SB_VERSION_NUM(sb) == XFS_SB_VERSION_5) {
+				printf(
+_("Cannot disable lazy-counters on V5 fs\n"));
+				exit(1);
+			}
 			sb->sb_features2 &= ~XFS_SB_VERSION2_LAZYSBCOUNTBIT;
 			sb->sb_bad_features2 &= ~XFS_SB_VERSION2_LAZYSBCOUNTBIT;
 			printf(_("Disabling lazy-counters\n"));


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