Re: [PATCH v2 1/8] io_uring/uring_cmd: add io_uring_cmd_import_fixed_full()

[Joanne Koong <joannelkoong@gmail.com>]

On Wed, Oct 29, 2025 at 7:01 AM Pavel Begunkov <asml.silence@gmail.com> wrote:
>
> On 10/27/25 22:28, Joanne Koong wrote:
> > Add an API for fetching the registered buffer associated with a
> > io_uring cmd. This is useful for callers who need access to the buffer
> > but do not have prior knowledge of the buffer's user address or length.
>
> Joanne, is it needed because you don't want to pass {offset,size}
> via fuse uapi? It's often more convenient to allocate and register
> one large buffer and let requests to use subchunks. Shouldn't be
> different for performance, but e.g. if you try to overlay it onto
> huge pages it'll be severely overaccounted.
>

Hi Pavel,

Yes, I was thinking this would be a simpler interface than the
userspace caller having to pass in the uaddr and size on every
request. Right now the way it is structured is that userspace
allocates a buffer per request, then registers all those buffers. On
the kernel side when it fetches the buffer, it'll always fetch the
whole buffer (eg offset is 0 and size is the full size).

Do you think it is better to allocate one large buffer and have the
requests use subchunks? My worry with this is that it would lead to
suboptimal cache locality when servers offload handling requests to
separate thread workers. From a code perspective it seems a bit
simpler to have each request have its own buffer, but it wouldn't be
much more complicated to have it all be part of one large buffer.

Right now, we are fetching the bvec iter every time there's a request
because of the possibility that the buffer might have been
unregistered (libfuse will not do this, but some other rogue userspace
program could). If we added a flag to tell io uring that attempts at
unregistration should return -EBUSY, then we could just fetch the bvec
iter once and use that for the lifetime of the server connection
instead of having to fetch it every request, and then when the
connection is aborted, we could unset the flag so that userspace can
then successfully unregister their buffers. Do you think this is a
good idea to have in io-uring? If this is fine to add then I'll add
this to v3.

Thanks,
Joanne

> --
> Pavel Begunkov
>