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

[John Garry]

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.

Least worst I can think if is to alloc some temp memory per-bio in 
nvme_ns_head_submit_bio() to hold this bdev pointer and original 
bi_private, set bio->bi_private to that memory, and then set bi_private 
back to original when we end the bio. But this is all crappy, especially 
just for partition diskstats.