linux-ext4 - Re: [PATCH e2fsprogs] UPDATED: ignore "safe" flag differences when fsck compares superblocks

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Date:	Fri, 25 Jan 2008 12:29:58 -0600
From:	Eric Sandeen <sandeen@...hat.com>
To:	ext4 development <linux-ext4@...r.kernel.org>
Subject: Re: [PATCH e2fsprogs] UPDATED: ignore "safe" flag differences when
 fsck compares superblocks

(updated for thinko: when proper flag *is* set on both primary & backup)

Recent e2fsprogs (1.40.3 and higher) fsck compares primary superblock to 
backups, and if things differ, it forces a full check.  However, the
kernel has a penchant for updating flags the first time a feature is
used - attributes, large files, etc.

However, it only updates these on the primary sb.  This then causes
the new e2fsck behavior to trigger a full check.  I think these flags
can be safely ignored on this check; having them set on the primary
but not the backups doesn't indicate corruption; if they're wrongly
set on the primary, really no damage is done, and if the backup is
used, but it doesn't have the flags set when it should, I'm pretty sure
e2fsck can cope with that.

I'll admit the patch below is not glamorous.  Any comments, either
on the style(sic) or the intent of the patch?

Thanks,

-Eric

Signed-off-by: Eric Sandeen <sandeen@...hat.com>

Index: e2fsprogs-1.40.4/e2fsck/super.c
===================================================================
--- e2fsprogs-1.40.4.orig/e2fsck/super.c
+++ e2fsprogs-1.40.4/e2fsck/super.c
@@ -814,10 +814,32 @@ int check_backup_super_block(e2fsck_t ct
 			continue;
 		}
 
-#define SUPER_DIFFERENT(x) (fs->super->x != tfs->super->x)
-		if (SUPER_DIFFERENT(s_feature_compat) ||
-		    SUPER_DIFFERENT(s_feature_incompat) ||
-		    SUPER_DIFFERENT(s_feature_ro_compat) ||
+		/*
+		 * A few flags are set on the fly by the kernel, but
+		 * only in the primary superblock.  They are safe
+	 	 * to copy even if they differ.
+	 	 */ 
+
+#define FEATURE_COMPAT_IGNORE		(EXT2_FEATURE_COMPAT_EXT_ATTR)
+#define FEATURE_RO_COMPAT_IGNORE	(EXT2_FEATURE_RO_COMPAT_LARGE_FILE| \
+					 EXT4_FEATURE_RO_COMPAT_DIR_NLINK)
+#define FEATURE_INCOMPAT_IGNORE		(EXT3_FEATURE_INCOMPAT_EXTENTS)
+
+#define SUPER_COMPAT_DIFFERENT(x)	\
+	(( fs->super->x & ~FEATURE_COMPAT_IGNORE) !=	\
+	 (tfs->super->x & ~FEATURE_COMPAT_IGNORE))
+#define SUPER_INCOMPAT_DIFFERENT(x)	\
+	(( fs->super->x & ~FEATURE_INCOMPAT_IGNORE) !=	\
+	 (tfs->super->x & ~FEATURE_INCOMPAT_IGNORE))
+#define SUPER_RO_COMPAT_DIFFERENT(x)	\
+	(( fs->super->x & ~FEATURE_RO_COMPAT_IGNORE) !=	\
+	 (tfs->super->x & ~FEATURE_RO_COMPAT_IGNORE))
+#define SUPER_DIFFERENT(x)		\
+	(fs->super->x != tfs->super->x)
+
+		if (SUPER_COMPAT_DIFFERENT(s_feature_compat) ||
+		    SUPER_INCOMPAT_DIFFERENT(s_feature_incompat) ||
+		    SUPER_RO_COMPAT_DIFFERENT(s_feature_ro_compat) ||
 		    SUPER_DIFFERENT(s_blocks_count) ||
 		    SUPER_DIFFERENT(s_inodes_count) ||
 		    memcmp(fs->super->s_uuid, tfs->super->s_uuid,


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