Re: [PATCH rdma-next v3 03/17] RDMA/core: Introduce generic buffer descriptor infrastructure for umem

[Jiri Pirko <jiri@resnulli.us>]

Tue, May 12, 2026 at 08:12:36PM +0200, jgg@ziepe.ca wrote:
>On Wed, May 06, 2026 at 04:14:34PM +0200, Jiri Pirko wrote:
>> Wed, May 06, 2026 at 03:37:57PM +0200, jgg@ziepe.ca wrote:
>> >On Mon, May 04, 2026 at 03:57:17PM +0200, Jiri Pirko wrote:
>> >
>> >> +/**
>> >> + * ib_umem_get - Canonical on-demand umem getter.
>> >> + * @device:        IB device.
>> >> + * @udata:         uverbs udata bundle (may be NULL).
>> >> + * @attr_id:       per-command UMEM attribute id; consulted if @udata is set.
>> >> + * @legacy_filler: optional command-specific legacy attr filler.
>> >> + *                 invoked if @udata is set.
>> >> + * @va_fallback:   if true, build a VA-typed desc with @addr.
>> >> + * @addr:          user VA, used if @va_fallback is true.
>> >> + * @size:          driver-required minimum length.
>> >> + * @access:        IB access flags forwarded to ib_umem_get_desc().
>> >> + *
>> >> + * Return: valid umem on success, ERR_PTR(...) on error, NULL
>> >> + * if no source produced a buffer (only possible when @va_fallback is false).
>> >> + */
>> >> +struct ib_umem *ib_umem_get(struct ib_device *device, struct ib_udata *udata,
>> >> +			    u16 attr_id,
>> >> +			    ib_umem_buf_desc_filler_t legacy_filler,
>> >> +			    bool va_fallback, u64 addr, size_t size, int access)
>> >
>> >I didn't try to look at what the drivers actually do, but I'm slightly
>> >surprised not to see an addr_size here? Is it the case the drivers
>> >don't have have a uhw->size to go along with their uhw->va?
>> 
>> "size_t size". What am I missing?
>
>size is the minimum length, not the actual length passed into the
>system call

Right, you're correct. Size is the post-pin minimum, and on
the VA fallback path it serves also as the pin length (we synthesize
desc.length = size when no attr/legacy filler matched).

There's no addr_size because no caller has a user-passed
length distinct from the driver-required minimum.
Drivers either have no length in their legacy ucmd (mlx4/mlx5 CQ, mlx5 QP,
the size is driver-computed from entries*cqe_size etc.) or they
pass ucmd.buf_size which serves as both (vmw_pvrdma, qedr, mlx5
SRQ, ...).

The legacy-filler path does carry a real user-passed
length (desc->length from UVERBS_ATTR_CREATE_CQ_BUFFER_LENGTH or
the per-attribute UMEM payload), and there size is purely the
post-pin check.

I'll tighten the kernel-doc to spell out the dual role:

  @size: minimum required umem length; also used as the pin
         length on the VA fallback path.

Makes sense?

[..]