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

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Message-ID: <c130133f-7c4c-4875-a850-1a8ac9ad4845@linux.ibm.com>
Date: Wed, 14 Feb 2024 15:08:34 +0530
From: Nilay Shroff <nilay@...ux.ibm.com>
To: John Garry <john.g.garry@...cle.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 2/13/24 17:22, John Garry wrote:
> 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?
If NABSPF is zero then there's no boundary and so we may not need to worry about IO crossing boundary.

Even though, the atomic boundary is not defined, this function doesn't allow atomic write crossing atomic_write_unit_max_bytes.
For instance, if AWUPF is 63 and an IO starts atomic write from logical block #32 and the number of logical blocks to be written
in this IO equals to #64 then it's not allowed. However if this same IO starts from logical block #0 then it's allowed.
So my point here's that can this restriction be avoided when atomic boundary is zero (or not defined)? 

Also, it seems that the restriction implemented for atomic write to succeed are very strict. For example, atomic-write can't
succeed if an IO starts from logical block #8 and the number of logical blocks to be written in this IO equals to #16. 
In this particular case, IO is well within atomic-boundary (if it's defined) and atomic-size-limit, so why do we NOT want to 
allow it? Is it intentional? I think, the spec doesn't mention about such limitation.

> 
>>
>> 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?
> 
Yes I will send this query to the NVMe patch in this series.

Thanks,
--Nilay