Re: [PATCH RFC 1/4] block: Make bdev_can_atomic_write() robust against mis-aligned bdev size

Hannes Reinecke
2024-09-13T08:36:38Z

On 9/12/24 17:22, John Garry wrote:
> On 12/09/2024 16:07, Christoph Hellwig wrote:
>>> We should do be able to, but with this patch we cannot. However, a
>>> misaligned partition would be very much unexpected.
>> Yes, misaligned partitions is very unexpected, but with large and
>> potentially unlimited atomic boundaries I would not expect the size
>> to always be aligned.  But then again at least in NVMe atomic writes
>> don't need to match the max size anyway, so I'm not entirely sure
>> what the problem actually is.
>
> Actually it's not an alignment issue, but a size issue.
>
> Consider a 3.5MB partition and atomic write max is 1MB. If we tried to
> atomic write 1MB at offset 3MB, then it would be truncated to a 0.5MB
> write.
>
> So maybe it is an application bug.
>
Hmm. Why don't we reject such an I/O? We cannot guarantee an atomic
write, so I think we should be perfectly fine to return an error to
userspace.

Cheers,

Hannes
--
Dr. Hannes Reinecke Kernel Storage Architect
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