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Message-ID: <a84ad9de-a274-4bdf-837a-03c38a32288a@oracle.com>
Date: Mon, 3 Jun 2024 12:38:57 +0100
From: John Garry <john.g.garry@...cle.com>
To: Hannes Reinecke <hare@...e.de>, 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, dchinner@...hat.com, jack@...e.cz
Cc: linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
linux-nvme@...ts.infradead.org, linux-fsdevel@...r.kernel.org,
tytso@....edu, jbongio@...gle.com, linux-scsi@...r.kernel.org,
ojaswin@...ux.ibm.com, linux-aio@...ck.org,
linux-btrfs@...r.kernel.org, io-uring@...r.kernel.org,
nilay@...ux.ibm.com, ritesh.list@...il.com, willy@...radead.org,
Himanshu Madhani <himanshu.madhani@...cle.com>
Subject: Re: [PATCH v7 4/9] block: Add core atomic write support
On 03/06/2024 10:26, Hannes Reinecke wrote:
>>
>> +static bool rq_straddles_atomic_write_boundary(struct request *rq,
>> + unsigned int front_adjust,
>> + unsigned int back_adjust)
>> +{
>> + unsigned int boundary = queue_atomic_write_boundary_bytes(rq->q);
>> + u64 mask, start_rq_pos, end_rq_pos;
>> +
>> + if (!boundary)
>> + return false;
>> +
>> + start_rq_pos = blk_rq_pos(rq) << SECTOR_SHIFT;
>> + end_rq_pos = start_rq_pos + blk_rq_bytes(rq) - 1;
>> +
>> + start_rq_pos -= front_adjust;
>> + end_rq_pos += back_adjust;
>> +
>> + mask = ~(boundary - 1);
>> +
>> + /* Top bits are different, so crossed a boundary */
>> + if ((start_rq_pos & mask) != (end_rq_pos & mask))
>> + return true;
>> +
>> + return false;
>> +}
>
> But isn't that precisely what 'chunk_sectors' is doing?
> IE ensuring that requests never cross that boundary?
>
> Q1: Shouldn't we rather use/modify/adapt chunk_sectors for this thing?
So you are saying that we can re-use blk_chunk_sectors_left() to
determine whether merging a bio/req would cross the boundary, right?
It seems ok in principle - we would just need to ensure that it is
watertight.
> Q2: If we don't, shouldn't we align the atomic write boundary to the
> chunk_sectors setting to ensure both match up?
Yeah, right. But we can only handle what HW tells.
The atomic write boundary is only relevant to NVMe. NVMe NOIOB - which
we use to set chunk_sectors - is an IO optimization hint, AFAIK. However
the atomic write boundary is a hard limit. So if NOIOB is not aligned
with the atomic write boundary - which seems unlikely - then the atomic
write boundary takes priority.
Thanks,
John
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