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

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Date: Thu, 21 Dec 2023 13:57:13 +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 12:48:24PM +0000, John Garry wrote:
>>> - 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,
>
> ok, I think that alignment is not so important, but we still need to 
> consider a minimum length per iovec, such that we will always be able to 
> fit a write of length atomic_write_unit_max in a bio.

I don't think you man a minim length per iovec for that, but a maximum
number of iovecs instead.  For SGL-capable devices that would be
BIO_MAX_VECS, otherwise 1.