linux-kernel - Re: [PATCH v3 09/15] block: Add checks to merging of atomic writes

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Message-ID: <76b6067f-109f-464c-97fb-fa519f3d1c56@linux.ibm.com>
Date: Mon, 12 Feb 2024 17:31:20 +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 09/15] block: Add checks to merging of atomic writes



On 2/12/24 16:50, John Garry wrote:
> I'm not sure what is going on with your mail client here.

Sorry for the inconvenience, I will check the settings.

>>
>> So is it a good idea to validate here whether we could potentially exceed
>>
>> the atomic-write-max-unit-size supported by device before we allow merging?
> 
> Note that we have atomic_write_max_bytes and atomic_write_max_unit_size, and they are not always the same thing.
> 
>>
>> In case we exceed the atomic-write-max-unit-size post merge then don't allow
>>
>> merging?
> 
> We check this elsewhere. I just expanded the normal check for max request size to cover atomic writes.
> 
> Normally we check that a merged request would not exceed max_sectors value, and this max_sectors value can be got from blk_queue_get_max_sectors().
> 
> So if you check a function like ll_back_merge_fn(), we have a merging size check:
> 
>     if (blk_rq_sectors(req) + bio_sectors(bio) >
>         blk_rq_get_max_sectors(req, blk_rq_pos(req))) {
>         req_set_nomerge(req->q, req);
>         return 0;
>     }
> 
> And here the blk_rq_get_max_sectors() -> blk_queue_get_max_sectors() call now also supports atomic writes (see patch #7):
OK got it. I think I have missed this part.

> 
> @@ -167,7 +167,16 @@ static inline unsigned get_max_io_size(struct bio *bio,
>  {
> ...
> 
> +    if (bio->bi_opf & REQ_ATOMIC)
> +        max_sectors = lim->atomic_write_max_sectors;
> +    else
> +        max_sectors = lim->max_sectors;
> 
> Note that we do not allow merging of atomic and non-atomic writes.
> 
Yeah 

Thanks,
--Nilay