linux-kernel - Re: [PATCH v2 00/16] block atomic writes

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Date: Thu, 21 Dec 2023 13:19:25 +0100
From: Christoph Hellwig <hch@....de>
To: John Garry <john.g.garry@...cle.com>
Cc: Christoph Hellwig <hch@....de>, "Darrick J. Wong" <djwong@...nel.org>,
	axboe@...nel.dk, kbusch@...nel.org, sagi@...mberg.me,
	jejb@...ux.ibm.com, martin.petersen@...cle.com,
	viro@...iv.linux.org.uk, brauner@...nel.org, dchinner@...hat.com,
	jack@...e.cz, linux-block@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-nvme@...ts.infradead.org,
	linux-xfs@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	tytso@....edu, jbongio@...gle.com, linux-scsi@...r.kernel.org,
	ming.lei@...hat.com, jaswin@...ux.ibm.com, bvanassche@....org
Subject: Re: [PATCH v2 00/16] block atomic writes

On Thu, Dec 21, 2023 at 09:49:35AM +0000, John Garry wrote:
> I noticed the NVMe patch to stop always setting virt boundary (thanks), but 
> I am struggling for the wording for iovecs rules. I'd like to reuse 
> iov_iter_is_aligned() to enforce any such rule.
>
> I am thinking:
> - ubuf / iovecs need to be PAGE-aligned
> - each iovec needs to be length of multiple of PAGE_SIZE
>
> But that does not work for total length < PAGE_SIZE.
>
> So then we could have:
> - ubuf / iovecs need to be PAGE-aligned
> - each iovec needs to be length of multiple of atomic_write_unit_min. If 
> total length > PAGE_SIZE, each iovec also needs to be a multiple of 
> PAGE_SIZE.
>
> I'd rather something simpler. Maybe it's ok.

If we decided to not support atomic writes on anything setting a virt
boundary we don't have to care about the alignment of each vector,
and IMHO we should do that as everything else would be a life in
constant pain.  If we really have a use case for atomic writes on
consumer NVMe devices we'll just have to limit it to a single iovec.