Re: [PATCH] block: use plug request list tail for one-shot backmerge attempt

[Christoph Hellwig <hch@infradead.org>]

On Wed, Jun 11, 2025 at 11:53:07AM -0600, Jens Axboe wrote:
> Yes we can't revert it, and honestly I would not want to even if that
> was an option. If the multi-queue case is particularly important, you
> could just do something ala the below - keep scanning until you a merge
> _could_ have happened but didn't. Ideally we'd want to iterate the plug
> list backwards and then we could keep the same single shot logic, where
> you only attempt one request that has a matching queue. And obviously we
> could just doubly link the requests, there's space in the request
> linkage code to do that. But that'd add overhead in general, I think
> it's better to shove a bit of that overhead to the multi-queue case.

Maybe byte the bullet and just make the request lists doubly linked?
Unlike the bio memory usage for request should not be quite as
critical.  Right now in my config the las cacheline in struct request
only has a single 8 byte field anyway, so in practive we won't even
bloat it.