Re: [PATCH RFC] nvme-multipath: fix diskstats for partitions

[John Garry <john.garry@linux.dev>]

On 7/24/26 13:45, Keith Busch wrote:
> On Fri, Jul 24, 2026 at 08:09:51AM +0100, John Garry wrote:
>> On 24/07/2026 06:01, Christoph Hellwig wrote:
>>> On Tue, Jul 21, 2026 at 11: 45: 53AM +0000, John Garry wrote: > Setting as an
>>> RFC as adding this extra bio field is not acceptable, but I > can't see how to
>>> lookup the original partition. In general it is not, but you should mark this a
>>>
>>>
>>> On Tue, Jul 21, 2026 at 11:45:53AM +0000, John Garry wrote:
>>>> Setting as an RFC as adding this extra bio field is not acceptable, but I
>>>> can't see how to lookup the original partition.
>>>
>>> In general it is not, but you should mark this a block patch so that
>>> Jens can better cream at you :)
>>>
>>> I'm also not sure that supporting per-partition diskstats on a multipath
>>> device makes too much sense, but then again there's a lot of setups
>>> that are crazy and actually used..
>>>
>>
>> It just seems to me that we should have same behaviour as if it were not
>> multipath.
>>
>> So we can't use bi_private as that can be set by original bio submitter. I
>> was thinking that this bdev pointer could be temp stashed in bi_next (as it
>> should be originally NULL), but that it dodgy and maybe won't even work.
> 
> Failover is corner case to consider here. We always reset the bio bdev
> to the part0, so your stats will be incorrect when that happens. I think
> you can quickly fix that in your proposal with the "bi_orig", though.
> 
> Can we just thread through partitions for the bio's block_device instead
> of assuming part0? I know the hidden path devices skip partition
> scanning, but maybe if we let it happen, then those will have the same
> partition setup as the head gendisk. Then we can go right to the
> disk->part_tbl for what we provide to bio_set_dev() for both submission
> and failover, and everything should work out from there.

I guess that we would just scan through the per-path gendisk->part_tbl 
and match somehow to lookup the partition. Maybe vs start address of 
bio->bi_bdev. Or is there a simpler (and quicker) way?