linux-kernel - Re: [PATCH v3 14/15] nvme: Support atomic writes

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Date: Wed, 14 Feb 2024 09:21:18 +0000
From: John Garry <john.g.garry@...cle.com>
To: Christoph Hellwig <hch@....de>
Cc: axboe@...nel.dk, kbusch@...nel.org, sagi@...mberg.me, jejb@...ux.ibm.com,
        martin.petersen@...cle.com, djwong@...nel.org, 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, ojaswin@...ux.ibm.com,
        bvanassche@....org, Alan Adamson <alan.adamson@...cle.com>
Subject: Re: [PATCH v3 14/15] nvme: Support atomic writes

On 14/02/2024 08:00, Christoph Hellwig wrote:
> On Tue, Feb 13, 2024 at 02:21:25PM +0000, John Garry wrote:
>>> Please also read through TP4098(a) and look at the MAM field.
>>
>> It's not public, AFAIK.
> 
> Oracle is a member, so you can take a look at it easily.  If we need
> it for Linux I can also work with the NVMe Board to release it.

What I really meant was that I prefer not to quote private TPs in public 
domain. I have the doc.

> 
>> And I don't think a feature which allows us to straddle boundaries is too
>> interesting really.
> 
> Without MAM=1 NVMe can't support atomic writes larger than
> AWUPF/NAWUPF, which is typically set to the indirection table
> size.  You're leaving a lot of potential unused with that.
> 

atomic_write_unit_max would always be dictated by min of boundary and 
AWUPF/NAWUPF. With MAM=1, we could increase atomic_write_max_bytes - but 
does it really help us? I mean, atomic_write_max_bytes only comes into 
play for merging - do we get much merging for NVMe transports? I am not 
sure.

Thanks,
John