Messages in this thread | | | Subject | Re: [PATCH] jbd2: avoid mount failed when commit block is partial submitted | From | "yebin (H)" <> | 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
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