Re: [PATCH v2 8/8] fuse: support io-uring registered buffers

[Bernd Schubert <bschubert@ddn.com>]

On 11/7/25 23:16, Bernd Schubert wrote:
> 
> 
> On 11/7/25 00:09, Joanne Koong wrote:
>> On Thu, Nov 6, 2025 at 11:48 AM Bernd Schubert <bernd@bsbernd.com> wrote:
>>>
>>> On 10/27/25 23:28, Joanne Koong wrote:
>>>> Add support for io-uring registered buffers for fuse daemons
>>>> communicating through the io-uring interface. Daemons may register
>>>> buffers ahead of time, which will eliminate the overhead of
>>>> pinning/unpinning user pages and translating virtual addresses for every
>>>> server-kernel interaction.
>>>>
>>>> To support page-aligned payloads, the buffer is structured such that the
>>>> payload is at the front of the buffer and the fuse_uring_req_header is
>>>> offset from the end of the buffer.
>>>>
>>>> To be backwards compatible, fuse uring still needs to support non-registered
>>>> buffers as well.
>>>>
>>>> Signed-off-by: Joanne Koong <joannelkoong@gmail.com>
>>>> ---
>>>>  fs/fuse/dev_uring.c   | 200 +++++++++++++++++++++++++++++++++---------
>>>>  fs/fuse/dev_uring_i.h |  27 +++++-
>>>>  2 files changed, 183 insertions(+), 44 deletions(-)
>>>>
>>>> diff --git a/fs/fuse/dev_uring.c b/fs/fuse/dev_uring.c
>>>> index c6b22b14b354..f501bc81f331 100644
>>>> --- a/fs/fuse/dev_uring.c
>>>> +++ b/fs/fuse/dev_uring.c
>>>>
>>>> +/*
>>>> + * Prepare fixed buffer for access. Sets up the payload iter and kmaps the
>>>> + * header.
>>>> + *
>>>> + * Callers must call fuse_uring_unmap_buffer() in the same scope to release the
>>>> + * header mapping.
>>>> + *
>>>> + * For non-fixed buffers, this is a no-op.
>>>> + */
>>>> +static int fuse_uring_map_buffer(struct fuse_ring_ent *ent)
>>>> +{
>>>> +     size_t header_size = sizeof(struct fuse_uring_req_header);
>>>> +     struct iov_iter iter;
>>>> +     struct page *header_page;
>>>> +     size_t count, start;
>>>> +     ssize_t copied;
>>>> +     int err;
>>>> +
>>>> +     if (!ent->fixed_buffer)
>>>> +             return 0;
>>>> +
>>>> +     err = io_uring_cmd_import_fixed_full(ITER_DEST, &iter, ent->cmd, 0);
>>>
>>> This seems to be a rather expensive call, especially as it gets
>>> called twice (during submit and fetch).
>>> Wouldn't be there be a possibility to check if the user buffer changed
>>> and then keep the existing iter? I think Caleb had a similar idea
>>> in patch 1/8.
>>
>> I think the best approach is to get rid of the call entirely by
>> returning -EBUSY to the server if it tries unregistering the buffers
>> while a connection is still alive. Then we would just have to set this
>> up once at registration time, and use that for the lifetime of the
>> connection. The discussion about this with Pavel is in [1] - I'm
>> planning to do this as a separate follow-up.
>>
>> [1] https://lore.kernel.org/linux-fsdevel/9f0debb1-ce0e-4085-a3fe-0da7a8fd76a6@gmail.com/
> 
> Hmm, I had seen this discussion, but I don't find anything about
> preventing unregistration?

Ah found it, sorry for the noise.