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

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Message-ID: <4a015015-ae7f-4eb5-ad00-420db5961d96@oracle.com>
Date: Thu, 12 Sep 2024 16:22:05 +0100
From: John Garry <john.g.garry@...cle.com>
To: Christoph Hellwig <hch@....de>
Cc: axboe@...nel.dk, song@...nel.org, yukuai3@...wei.com, kbusch@...nel.org,
        sagi@...mberg.me, James.Bottomley@...senPartnership.com,
        martin.petersen@...cle.com, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-raid@...r.kernel.org,
        linux-nvme@...ts.infradead.org, linux-scsi@...r.kernel.org
Subject: Re: [PATCH RFC 1/4] block: Make bdev_can_atomic_write() robust
 against mis-aligned bdev size

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.

> 
>> I could also just reject any truncation on the atomic write in fops. Maybe
>> that is better.