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

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Message-ID: <1b4d5860-8044-4a1f-a801-1c69327076c1@linux.ibm.com>
Date: Wed, 14 Feb 2024 22:15:50 +0530
From: Nilay Shroff <nilay@...ux.ibm.com>
To: John Garry <john.g.garry@...cle.com>
Cc: alan.adamson@...cle.com, 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 14/15] nvme: Support atomic writes



On 2/14/24 18:32, John Garry wrote:
> On 14/02/2024 12:27, Nilay Shroff wrote:
>>
>>
>>
>>> Use following method to calculate limits:
>>
>>> atomic_write_max_bytes = flp2(NAWUPF ?: AWUPF)
>>
> 
> You still need to fix that mail client to not add extra blank lines.
Yes, I am working on it. I hope it's solved now. 
> 
>>> atomic_write_unit_min = logical_block_size
>>
>>> atomic_write_unit_max = flp2(NAWUPF ?: AWUPF)
>>
>>> atomic_write_boundary = NABSPF
>>
>>
>>
>> In case the device doesn't support namespace atomic boundary size (i.e. NABSPF
>>
>> is zero) then while merging atomic block-IO we should allow merge.
>>
>>  
>> For example, while front/back merging the atomic block IO, we check whether
>>
>> boundary is defined or not. In case if boundary is not-defined (i.e. it's zero)
>>
>> then we simply reject merging ateempt (as implemented in
>>
>> rq_straddles_atomic_write_boundary()).
> 
> Are you sure about that? In rq_straddles_atomic_write_boundary(), if boundary == 0, then we return false, i.e. there is no boundary, so we can never be crossing it.
> 
> static bool rq_straddles_atomic_write_boundary(struct request *rq,
> unsigned int front,
> unsigned int back)
> {
>     unsigned int boundary = queue_atomic_write_boundary_bytes(rq->q);
>     unsigned int mask, imask;
>     loff_t start, end;
> 
>     if (!boundary)
>         return false;
> 
>     ...
> }
> 
> And then will not reject a merge for that reason, like:
> 
> int ll_back_merge_fn(struct request *req, struct bio *bio, unsigned int nr_segs)
> {
>     ...
> 
>     if (req->cmd_flags & REQ_ATOMIC) {
>         if (rq_straddles_atomic_write_boundary(req,
>             0, bio->bi_iter.bi_size)) {
>             return 0;
>         }
>     }
> 
>     return ll_new_hw_segment(req, bio, nr_segs);
> }
> 
> 
Aargh, you are right. I see that if rq_straddles_atomic_write_boundary() returns true then we avoid merge otherwise the merge is attempted. My bad...

Thanks,
--Nilay