Date: Wed, 3 Apr 2024 12:11:22 +0200
From: Jan Kara <jack@...e.cz>
To: Theodore Ts'o <tytso@....edu>
Cc: Jan Kara <jack@...e.cz>, Ye Bin <yebin10@...wei.com>,
	adilger.kernel@...ger.ca, linux-ext4@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] jbd2: avoid mount failed when commit block is partial
 submitted

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@...wei.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...

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
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR

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