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

[Bernd Schubert <bernd@bsbernd.com>]

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?


Thanks,
Bernd