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

[Christoph Hellwig <hch@infradead.org>]

On Thu, Jun 12, 2025 at 05:49:16AM -0600, Jens Axboe wrote:
> > 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.
> 
> The space isn't a concern, as you found as well. It's the fact that
> doubly linked lists suck in terms of needing to touch both prev
> and next for removal.

But is that actually a concern here?  If you look at my patch we can
now use the list_cut helper for the queue_rqs submission sorting,
and for the actual submission we walk each request anyway (and might
get along without removal entirely if we dare to leave the dangling
pointers around).  The multi-queue dispatch could probably use the
cut as well.  For the cached rqs and completion  we could stіck to the
singly linked list as they don't really mix up with the submission
IFF that shows up as an issue there.