Ran into an ext4 regression when testing upgrades to 5.9-rc kernels:
Commit e7bfb5c9bb3d ("ext4: handle add_system_zone() failure in
ext4_setup_system_zone()") breaks mounting of read-only ext4 filesystems
with intentionally overlapping bitmap blocks.
On an always-read-only filesystem explicitly marked with
EXT4_FEATURE_RO_COMPAT_SHARED_BLOCKS, prior to that commit, it's safe to
point all the block and inode bitmaps to a single block of all 1s,
because a read-only filesystem will never allocate or free any blocks or
inodes.
However, after that commit, the block validity check rejects such
filesystems with -EUCLEAN and "failed to initialize system zone (-117)".
This causes systems that previously worked correctly to fail when
upgrading to v5.9-rc2 or later.
This was obviously a bugfix, and I'm not suggesting that it should be
reverted; it looks like this effectively worked by accident before,
because the block_validity check wasn't fully functional. However, this
does break real systems, and I'd like to get some kind of regression fix
in before 5.9 final if possible. I think it would suffice to make
block_validity default to false if and only if
EXT4_FEATURE_RO_COMPAT_SHARED_BLOCKS is set.
Does that seem like a reasonable fix?
Here's a quick sketch of a patch, which I've tested and confirmed to
work:
----- 8< -----
Subject: [PATCH] Fix ext4 regression in v5.9-rc2 on ro fs with overlapped bitmaps
Commit e7bfb5c9bb3d ("ext4: handle add_system_zone() failure in
ext4_setup_system_zone()") breaks mounting of read-only ext4 filesystems
with intentionally overlapping bitmap blocks.
On an always-read-only filesystem explicitly marked with
EXT4_FEATURE_RO_COMPAT_SHARED_BLOCKS, prior to that commit, it's safe to
point all the block and inode bitmaps to a single block of all 1s,
because a read-only filesystem will never allocate or free any blocks or
inodes.
However, after that commit, the block validity check rejects such
filesystems with -EUCLEAN and "failed to initialize system zone (-117)".
This causes systems that previously worked correctly to fail when
upgrading to v5.9-rc2 or later.
Fix this by defaulting block_validity to off when
EXT4_FEATURE_RO_COMPAT_SHARED_BLOCKS is set.
Signed-off-by: Josh Triplett <josh@xxxxxxxxxxxxxxxx>
Fixes: e7bfb5c9bb3d ("ext4: handle add_system_zone() failure in ext4_setup_system_zone()")
---
fs/ext4/ext4.h | 2 ++
fs/ext4/super.c | 3 ++-
2 files changed, 4 insertions(+), 1 deletion(-)
diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h
index 523e00d7b392..7874028fa864 100644
--- a/fs/ext4/ext4.h
+++ b/fs/ext4/ext4.h
@@ -1834,6 +1834,7 @@ static inline bool ext4_verity_in_progress(struct inode *inode)
#define EXT4_FEATURE_RO_COMPAT_METADATA_CSUM 0x0400
#define EXT4_FEATURE_RO_COMPAT_READONLY 0x1000
#define EXT4_FEATURE_RO_COMPAT_PROJECT 0x2000
+#define EXT4_FEATURE_RO_COMPAT_SHARED_BLOCKS 0x4000
#define EXT4_FEATURE_RO_COMPAT_VERITY 0x8000
#define EXT4_FEATURE_INCOMPAT_COMPRESSION 0x0001
@@ -1930,6 +1931,7 @@ EXT4_FEATURE_RO_COMPAT_FUNCS(bigalloc, BIGALLOC)
EXT4_FEATURE_RO_COMPAT_FUNCS(metadata_csum, METADATA_CSUM)
EXT4_FEATURE_RO_COMPAT_FUNCS(readonly, READONLY)
EXT4_FEATURE_RO_COMPAT_FUNCS(project, PROJECT)
+EXT4_FEATURE_RO_COMPAT_FUNCS(shared_blocks, SHARED_BLOCKS)
EXT4_FEATURE_RO_COMPAT_FUNCS(verity, VERITY)
EXT4_FEATURE_INCOMPAT_FUNCS(compression, COMPRESSION)
diff --git a/fs/ext4/super.c b/fs/ext4/super.c
index ea425b49b345..f57a7e966e44 100644
--- a/fs/ext4/super.c
+++ b/fs/ext4/super.c
@@ -3954,7 +3954,8 @@ static int ext4_fill_super(struct super_block *sb, void *data, int silent)
else
set_opt(sb, ERRORS_RO);
/* block_validity enabled by default; disable with noblock_validity */
- set_opt(sb, BLOCK_VALIDITY);
+ if (!ext4_has_feature_shared_blocks(sb))
+ set_opt(sb, BLOCK_VALIDITY);
if (def_mount_opts & EXT4_DEFM_DISCARD)
set_opt(sb, DISCARD);
