linux-kernel - Re: [PATCH v3 10/15] block: Add fops atomic write support

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Date: Tue, 13 Feb 2024 11:52:32 +0000
From: John Garry <john.g.garry@...cle.com>
To: Nilay Shroff <nilay@...ux.ibm.com>
Cc: axboe@...nel.dk, brauner@...nel.org, bvanassche@....org,
        dchinner@...hat.com, djwong@...nel.org, hch@....de, jack@...e.cz,
        jbongio@...gle.com, jejb@...ux.ibm.com, kbusch@...nel.org,
        linux-block@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-nvme@...ts.infradead.org,
        linux-scsi@...r.kernel.org, linux-xfs@...r.kernel.org,
        martin.petersen@...cle.com, ming.lei@...hat.com, ojaswin@...ux.ibm.com,
        sagi@...mberg.me, tytso@....edu, viro@...iv.linux.org.uk
Subject: Re: [PATCH v3 10/15] block: Add fops atomic write support

On 13/02/2024 11:08, Nilay Shroff wrote:
>> It's relied that atomic_write_unit_max is <= atomic_write_boundary and both are a power-of-2. Please see the NVMe patch, which this is checked. Indeed, it would not make sense if atomic_write_unit_max > atomic_write_boundary (when non-zero).
>>
>> So if the write is naturally aligned and its size is <= atomic_write_unit_max, then it cannot be straddling a boundary.
> Ok fine but in case the device doesn't support namespace atomic boundary size (i.e. NABSPF is zero) then still do we need
> to restrict IO which crosses the atomic boundary?

Is there a boundary if NABSPF is zero?

> 
> I am quoting this from NVMe spec (Command Set Specification, revision 1.0a, Section 2.1.4.3) :
> "To ensure backwards compatibility, the values reported for AWUN, AWUPF, and ACWU shall be set such that
> they  are  supported  even  if  a  write  crosses  an  atomic  boundary.  If  a  controller  does  not
> guarantee atomicity across atomic boundaries, the controller shall set AWUN, AWUPF, and ACWU to 0h (1 LBA)."

How about respond to the NVMe patch in this series, asking this question?

I have my idea on how the boundary is determined, but I think that the 
spec could be made clearer.

Thanks,
John