linux-kernel - Re: [PATCH 21/21] nvme: Support atomic writes

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Date:   Thu, 5 Oct 2023 16:05:50 +0100
From:   John Garry <john.g.garry@...cle.com>
To:     Pankaj Raghav <p.raghav@...sung.com>,
        Alan Adamson <alan.adamson@...cle.com>
Cc:     axboe@...nel.dk, kbusch@...nel.org, hch@....de, sagi@...mberg.me,
        jejb@...ux.ibm.com, martin.petersen@...cle.com, djwong@...nel.org,
        viro@...iv.linux.org.uk, brauner@...nel.org,
        chandan.babu@...cle.com, dchinner@...hat.com,
        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-api@...r.kernel.org
Subject: Re: [PATCH 21/21] nvme: Support atomic writes

On 05/10/2023 14:32, Pankaj Raghav wrote:
>>> te_unit_[min| max]_sectors expects sectors (512 bytes unit)
>>> as input but no conversion is done here from device logical block size
>>> to SECTORs.
>> Yeah, you are right. I think that we can just use:
>>
>> blk_queue_atomic_write_unit_max_sectors(disk->queue,
>> atomic_bs >> SECTOR_SHIFT);
>>
> Makes sense.
> I still don't grok the difference between max_bytes and unit_max_sectors here.
> (Maybe NVMe spec does not differentiate it?)

I think that max_bytes does not need to be a power-of-2 and could be 
relaxed.

Having said that, max_bytes comes into play for merging of bios - so if 
we are in a scenario with no merging, then may a well leave 
atomic_write_max_bytes == atomic_write_unit_max.

But let us check this proposal to relax.

> 
> I assume min_sectors should be as follows instead of setting it to 1 (512 bytes)?
> 
> blk_queue_atomic_write_unit_min_sectors(disk->queue, bs >> SECTORS_SHIFT);

Yeah, right, we want unit_min to be the logical block size.

Thanks,
John