Re: [PATCH] jbd2: avoid mount failed when commit block is partial submitted

[yebin (H)]

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@xxxxxxxxxx>
> > > 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