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From: Josh Triplett <josh@joshtriplett.org>
To: Linus Torvalds <torvalds@linux-foundation.org>,
	Theodore Ts'o <tytso@mit.edu>,
	Andreas Dilger <adilger.kernel@dilger.ca>,
	Jan Kara <jack@suse.com>
Cc: Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	linux-ext4@vger.kernel.org
Subject: ext4 regression in v5.9-rc2 from e7bfb5c9bb3d on ro fs with overlapped bitmaps
Date: Mon, 5 Oct 2020 01:14:54 -0700	[thread overview]
Message-ID: <20201005081454.GA493107@localhost> (raw)
In-Reply-To: <CAHk-=wj-H5BYCU_kKiOK=B9sN3BtRzL4pnne2AJPyf54nQ+d=w@mail.gmail.com>

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@joshtriplett.org>
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);
 

  reply	other threads:[~2020-10-05  8:22 UTC|newest]

Thread overview: 35+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-10-04 23:17 Linux 5.9-rc8 Linus Torvalds
2020-10-05  8:14 ` Josh Triplett [this message]
2020-10-05  9:46   ` ext4 regression in v5.9-rc2 from e7bfb5c9bb3d on ro fs with overlapped bitmaps Jan Kara
2020-10-05 10:16     ` Josh Triplett
2020-10-05 16:19       ` Jan Kara
2020-10-05 16:20   ` Jan Kara
2020-10-05 17:36   ` Darrick J. Wong
2020-10-06  0:04     ` Theodore Y. Ts'o
2020-10-06  0:32     ` Josh Triplett
2020-10-06  2:51       ` Darrick J. Wong
2020-10-06  3:18         ` Theodore Y. Ts'o
2020-10-06  5:03           ` Josh Triplett
2020-10-06  6:03             ` Josh Triplett
2020-10-06 13:35             ` Theodore Y. Ts'o
2020-10-07  8:03               ` Josh Triplett
2020-10-07 14:32                 ` Theodore Y. Ts'o
2020-10-07 20:14                   ` Josh Triplett
2020-10-08  2:10                     ` Theodore Y. Ts'o
2020-10-08 17:54                       ` Darrick J. Wong
2020-10-08 22:38                         ` Josh Triplett
2020-10-09  2:54                           ` Darrick J. Wong
2020-10-09 19:08                             ` Josh Triplett
2020-10-08 22:22                       ` Josh Triplett
2020-10-09 14:37                         ` Theodore Y. Ts'o
2020-10-09 20:30                           ` Josh Triplett
2021-01-10 18:41                           ` Malicious fs images was " Pavel Machek
2021-01-11 18:51                             ` Darrick J. Wong
2021-01-11 19:39                               ` Eric Biggers
2021-01-12 21:43                             ` Theodore Ts'o
2021-01-12 22:28                               ` Pavel Machek
2021-01-13  5:09                                 ` Theodore Ts'o
2020-10-08  2:57                     ` Andreas Dilger
2020-10-08 19:12                       ` Josh Triplett
2020-10-08 19:25                         ` Andreas Dilger
2020-10-08 22:28                           ` Josh Triplett

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