Re: [PATCH v7 4/9] block: Add core atomic write support

[John Garry <john.g.garry@oracle.com>]

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