Re: [PATCH v2 2/9] io_uring: add infra for importing vectored reg buffers

[Jens Axboe <axboe@kernel.dk>]

On 3/7/25 7:07 AM, Pavel Begunkov wrote:
> On 3/4/25 15:40, Pavel Begunkov wrote:
>> Add io_import_reg_vec(), which will be responsible for importing
>> vectored registered buffers. iovecs are overlapped with the resulting
>> bvec in memory, which is why the iovec is expected to be padded in
>> iou_vec.
>>
>> Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
>> ---
> ...
>> +int io_import_reg_vec(int ddir, struct iov_iter *iter,
>> +            struct io_kiocb *req, struct iou_vec *vec,
>> +            unsigned nr_iovs, unsigned iovec_off,
>> +            unsigned issue_flags)
>> +{
>> +    struct io_rsrc_node *node;
>> +    struct io_mapped_ubuf *imu;
>> +    struct iovec *iov;
>> +    unsigned nr_segs;
>> +
>> +    node = io_find_buf_node(req, issue_flags);
>> +    if (!node)
>> +        return -EFAULT;
>> +    imu = node->buf;
>> +    if (imu->is_kbuf)
>> +        return -EOPNOTSUPP;
>> +    if (!(imu->dir & (1 << ddir)))
>> +        return -EFAULT;
>> +
>> +    iov = vec->iovec + iovec_off;
>> +    nr_segs = io_estimate_bvec_size(iov, nr_iovs, imu);
> 
> if (sizeof(struct bio_vec) > sizeof(struct iovec)) {
>     size_t entry_sz = sizeof(struct iovec);
>     size_t bvec_bytes = nr_segs * sizeof(struct bio_vec);
>     size_t iovec_off = (bvec_bytes + entry_sz - 1) / entry_sz;
> 
>     nr_segs += iovec_off;
> }
> 
> How about fixing it up like this for now? Instead of overlapping
> bvec with iovec, it'd put them back to back and waste some memory
> on 32bit.
> 
> I can try to make it a bit tighter, remove the if and let
> the compiler to optimise it into no-op for x64, or allocate
> max(bvec, iovec) * nr and see where it leads. But in either
> way IMHO it's better to be left until I get more time.

I think that looks reasonable. Nobody cares about this for 32-bit,
outside of needing to work obviously.

-- 
Jens Axboe