Re: [PATCH] io_uring/rsrc: send exact nr_segs for fixed buffer

[Pavel Begunkov <asml.silence@gmail.com>]

On 4/16/25 21:01, Jens Axboe wrote:
> On 4/16/25 1:57 PM, Nitesh Shetty wrote:
...
>>>                  /*
>>> @@ -1073,7 +1075,6 @@ static int io_import_fixed(int ddir, struct iov_iter *iter,
>>>                   * since we can just skip the first segment, which may not
>>>                   * be folio_size aligned.
>>>                   */
>>> -               const struct bio_vec *bvec = imu->bvec;
>>>
>>>                  /*
>>>                   * Kernel buffer bvecs, on the other hand, don't necessarily
>>> @@ -1099,6 +1100,27 @@ static int io_import_fixed(int ddir, struct iov_iter *iter,
>>>                  }
>>>          }
>>>
>>> +       /*
>>> +        * Offset trimmed front segments too, if any, now trim the tail.
>>> +        * For is_kbuf we'll iterate them as they may be different sizes,
>>> +        * otherwise we can just do straight up math.
>>> +        */
>>> +       if (len + offset < imu->len) {
>>> +               bvec = iter->bvec;
>>> +               if (imu->is_kbuf) {
>>> +                       while (len > bvec->bv_len) {
>>> +                               len -= bvec->bv_len;
>>> +                               bvec++;
>>> +                       }
>>> +                       iter->nr_segs = bvec - iter->bvec;
>>> +               } else {
>>> +                       size_t vec_len;
>>> +
>>> +                       vec_len = bvec->bv_offset + iter->iov_offset +
>>> +                                       iter->count + ((1UL << folio_shift) - 1);
>>> +                       iter->nr_segs = vec_len >> folio_shift;
>>> +               }
>>> +       }
>>>          return 0;
>>>   }
>> This might not be needed for is_kbuf , as they already update nr_seg
>> inside iov_iter_advance.
> 
> How so? If 'offset' is true, then yes it'd skip the front, but it
> doesn't skip the end part. And if 'offset' is 0, then no advancing is
> done in the first place - which does make sense, as it's just advancing
> from the front.
> 
>> How about changing something like this ?
> 
> You can't hide this in the if (offset) section...

Should we just make it saner first? Sth like these 3 completely
untested commits

https://github.com/isilence/linux/commits/rsrc-import-cleanup/

And then it'll become

nr_segs = ALIGN(offset + len, 1UL << folio_shift);

-- 
Pavel Begunkov