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Subject: Re: [PATCH] jbd2: avoid mount failed when commit block is partial
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To: Jan Kara <jack@suse.cz>, Theodore Ts'o <tytso@mit.edu>
References: <20240402090951.527619-1-yebin10@huawei.com>
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CC: <adilger.kernel@dilger.ca>, <linux-ext4@vger.kernel.org>,
	<linux-kernel@vger.kernel.org>
From: "yebin (H)" <yebin10@huawei.com>
Message-ID: <6611F8D5.3030403@huawei.com>
Date: Sun, 7 Apr 2024 09:37:25 +0800
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On 2024/4/3 18:11, Jan Kara wrote:
> On Tue 02-04-24 23:37:42, Theodore Ts'o wrote:
>> On Tue, Apr 02, 2024 at 03:42:40PM +0200, Jan Kara wrote:
>>> On Tue 02-04-24 17:09:51, Ye Bin wrote:
>>>> We encountered a problem that the file system could not be mounted in
>>>> the power-off scenario. The analysis of the file system mirror shows that
>>>> only part of the data is written to the last commit block.
>>>> To solve above issue, if commit block checksum is incorrect, check the next
>>>> block if has valid magic and transaction ID. If next block hasn't valid
>>>> magic or transaction ID then just drop the last transaction ignore checksum
>>>> error. Theoretically, the transaction ID maybe occur loopback, which may cause
>>>> the mounting failure.
>>>>
>>>> Signed-off-by: Ye Bin <yebin10@huawei.com>
>>> So this is curious. The commit block data is fully within one sector and
>>> the expectation of the journaling is that either full sector or nothing is
>>> written. So what kind of storage were you using that it breaks these
>>> expectations?
>> I suppose if the physical sector size is 512 bytes, and the file
>> system block is 4k, I suppose it's possible that on a crash, that part
>> of the 4k commit block could be written.
> I was thinking about that as well but the commit block looks like:
>
> truct commit_header {
>          __be32          h_magic;
>          __be32          h_blocktype;
>          __be32          h_sequence;
>          unsigned char   h_chksum_type;
>          unsigned char   h_chksum_size;
>          unsigned char   h_padding[2];
>          __be32          h_chksum[JBD2_CHECKSUM_BYTES];
>          __be64          h_commit_sec;
>          __be32          h_commit_nsec;
> };
>
> where JBD2_CHECKSUM_BYTES is 8. So all the data in the commit block
> including the checksum is in the first 60 bytes. Hence I would be really
> surprised if some storage can tear that...
This issue has been encountered a few times in the context of eMMC 
devices. The vendor
has confirmed that only 512-byte atomicity can be ensured in the firmware.
Although the valid data is only 60 bytes, the entire commit block is 
used for calculating
the checksum.
jbd2_commit_block_csum_verify:
...
calculated = jbd2_chksum(j, j->j_csum_seed, buf, j->j_blocksize);
...
>
> Hence either Ye Bin is running on some really exotic storage or the storage
> / CPU in fact flipped bits somewhere so that the checksum didn't match or
> the commit block was in fact not written now but it was a leftover from
> previous journal use and h_sequence happened to match. Very unlikely but
> depending on how exactly they do their powerfail testing I can imagine it
> would be possible with enough tries...
>
>> In *practice* though, this
>> is super rare.  That's because on many modern HDD's, the physical
>> sector size is 4k (because the ECC overhead is much lower), even if
>> the logical sector size is 512 byte (for Windows 98 compatibility).
>> And even on HDD's where the physical sector size is really 512 bytes,
>> the way the sectors are laid out in a serpentine fashion, it is
>> *highly* likely that 4k write won't get torn.
>>
>> And while this is *possible*, it's also possible that some kind of I/O
>> transfer error --- such as some bit flips which breaks the checksum on
>> the commit block, but also trashes the tid of the subsequent block,
>> such that your patch gets tricked into thinking that this is the
>> partial last commit, when in fact it's not the last commit, thus
>> causing the journal replay abort early.  If that's case, it's much
>> safer to force fsck to be run to detect any inconsistency that might
>> result.
> Yeah, I agree in these cases of a corrupted journal it seems dangerous to
> just try to continue without fsck based on some heuristics.
>
> 								Honza