On Mon, Jun 9, 2025 at 7:07 PM Ming Lei <ming.lei@xxxxxxxxxx> wrote:
>
> On Mon, Jun 09, 2025 at 03:29:34PM -0700, Caleb Sander Mateos wrote:
> > On Mon, Jun 9, 2025 at 5:14 AM Ming Lei <ming.lei@xxxxxxxxxx> wrote:
> > >
> > > Document recently merged feature auto buffer registration(UBLK_F_AUTO_BUF_REG).
> > >
> > > Signed-off-by: Ming Lei <ming.lei@xxxxxxxxxx>
> >
> > Thanks, this is a nice explanation. Just a few suggestions.
> >
> > > ---
> > > Documentation/block/ublk.rst | 67 ++++++++++++++++++++++++++++++++++++
> > > 1 file changed, 67 insertions(+)
> > >
> > > diff --git a/Documentation/block/ublk.rst b/Documentation/block/ublk.rst
> > > index c368e1081b41..16ffca54eed4 100644
> > > --- a/Documentation/block/ublk.rst
> > > +++ b/Documentation/block/ublk.rst
> > > @@ -352,6 +352,73 @@ For reaching best IO performance, ublk server should align its segment
> > > parameter of `struct ublk_param_segment` with backend for avoiding
> > > unnecessary IO split, which usually hurts io_uring performance.
> > >
> > > +Auto Buffer Registration
> > > +------------------------
> > > +
> > > +The ``UBLK_F_AUTO_BUF_REG`` feature automatically handles buffer registration
> > > +and unregistration for I/O requests, which simplifies the buffer management
> > > +process and reduces overhead in the ublk server implementation.
> > > +
> > > +This is another feature flag for using zero copy, and it is compatible with
> > > +``UBLK_F_SUPPORT_ZERO_COPY``.
> > > +
> > > +Feature Overview
> > > +~~~~~~~~~~~~~~~~
> > > +
> > > +This feature automatically registers request buffers to the io_uring context
> > > +before delivering I/O commands to the ublk server and unregisters them when
> > > +completing I/O commands. This eliminates the need for manual buffer
> > > +registration/unregistration via ``UBLK_IO_REGISTER_IO_BUF`` and
> > > +``UBLK_IO_UNREGISTER_IO_BUF`` commands, then IO handling in ublk server
> > > +can avoid dependency on the two uring_cmd operations.
> > > +
> > > +This way not only simplifies ublk server implementation, but also makes
> > > +concurrent IO handling becomes possible.
> >
> > I'm not sure what "concurrent IO handling" refers to. Any ublk server
> > can handle incoming I/O requests concurrently, regardless of what
> > features it has enabled. Do you mean it avoids the need for linked
> > io_uring requests to properly order buffer registration and
> > unregistration with the I/O operations using the registered buffer?
>
> Yes, if io_uring OPs depends on buffer registering & unregistering, these
> OPs can't be issued concurrently any more, that is one io_uring constraint.
>
> I will add the above words.
>
> >
> > > +
> > > +Usage Requirements
> > > +~~~~~~~~~~~~~~~~~~
> > > +
> > > +1. The ublk server must create a sparse buffer table on the same ``io_ring_ctx``
> > > + used for ``UBLK_IO_FETCH_REQ`` and ``UBLK_IO_COMMIT_AND_FETCH_REQ``.
> > > +
> > > +2. If uring_cmd is issued on a different ``io_ring_ctx``, manual buffer
> > > + unregistration is required.
> >
> > nit: don't think this needs to be a separate point, could be combined with (1).
>
> OK.
>
> >
> > > +
> > > +3. Buffer registration data must be passed via uring_cmd's ``sqe->addr`` with the
> > > + following structure::
> >
> > nit: extra ":"
>
> In reStructuredText (reST), the double colon :: serves as a literal block marker to
> indicate preformatted text.
>
> >
> > > +
> > > + struct ublk_auto_buf_reg {
> > > + __u16 index; /* Buffer index for registration */
> > > + __u8 flags; /* Registration flags */
> > > + __u8 reserved0; /* Reserved for future use */
> > > + __u32 reserved1; /* Reserved for future use */
> > > + };
> >
> > Suggest using ublk_auto_buf_reg_to_sqe_addr()? Otherwise, it seems
> > ambiguous how this struct is "passed" in sqe->addr.
>
> OK
>
> >
> > > +
> > > +4. All reserved fields in ``ublk_auto_buf_reg`` must be zeroed.
> > > +
> > > +5. Optional flags can be passed via ``ublk_auto_buf_reg.flags``.
> > > +
> > > +Fallback Behavior
> > > +~~~~~~~~~~~~~~~~~
> > > +
> > > +When ``UBLK_AUTO_BUF_REG_FALLBACK`` is enabled:
> > > +
> > > +1. If auto buffer registration fails:
> >
> > I would switch these. Both (1) and (2) refer to when auto buffer
> > registration fails. So I would expect something like:
> >
> > If auto buffer registration fails:
> >
> > 1. When ``UBLK_AUTO_BUF_REG_FALLBACK`` is enabled:
> > ...
> > 2. If fallback is not enabled:
> > ...
> >
> > > + - The uring_cmd is completed
> >
> > Maybe add "without registering the request buffer"?
> >
> > > + - ``UBLK_IO_F_NEED_REG_BUF`` is set in ``ublksrv_io_desc.op_flags``
> > > + - The ublk server must manually register the buffer
> >
> > Only if it wants a registered buffer for the ublk request. Technically
> > the ublk server could decide to fall back on user-copy, for example.
>
> Good catch!
>
> >
> > > +
> > > +2. If fallback is not enabled:
> > > + - The ublk I/O request fails silently
> >
> > "silently" is a bit ambiguous. It's certainly not silent to the
> > application submitting the ublk I/O. Maybe say that the ublk I/O
> > request fails and no uring_cmd is completed to the ublk server?
>
> Yes, but the document focus on ublk side, and the client is generic
> for every driver, so I guess it may be fine.
>
> >
> > > +
> > > +Limitations
> > > +~~~~~~~~~~~
> > > +
> > > +- Requires same ``io_ring_ctx`` for all operations
> >
> > Another limitation that prevents us from adopting the auto buffer
> > registration feature is the need to reserve a unique buffer table
> > index for every ublk tag on the io_ring_ctx. Since the io_ring_ctx
> > buffer table has a max size of 16K (could potentially be increased to
> > 64K), this limit is easily reached when there are a large number of
> > ublk devices or the ublk queue depth is large. I think we could remove
> > this limitation in the future by adding support for allocating buffer
> > indices on demand, analogous to IORING_FILE_INDEX_ALLOC.
>
> OK.
>
> But I guess it isn't big deal in reality since the task context should
> be saturated easily with so big setting.
I don't know about your "reality" but it's certainly a big deal for us :)
To reduce contention on the blk-mq queues for the application
submitting I/O to the ublk devices, we want a large number of queues
for each ublk device. But we also want a large queue depth for each
individual queue to avoid the async request allocation path in case
any one application thread issues a lot of concurrent I/O to a single
ublk device. And we have 128 ublk devices, which again all want large
queue depths in case the application sends a lot of I/O to a single
ublk device. The result is that concurrently each ublk server thread
fetches 512K ublk I/Os, which is significantly above the io_ring_ctx
buffer table limit.
Best,
Caleb
