Re: [PATCH v6 5/6] fuse: add zero-copy over io-uring

On Thu, Aug 13, 2026 at 10:01 AM Bernd Schubert <[email protected]> wrote:
>
>
>
> On 8/13/26 01:19, Joanne Koong wrote:
> > On Wed, Aug 12, 2026 at 3:30 PM Bernd Schubert <[email protected]> wrote:
> >>
> >>> diff --git a/fs/fuse/dev.c b/fs/fuse/dev.c
> >>> index d50792162d8b..3d1910a9db52 100644
> >>> --- a/fs/fuse/dev.c
> >>> +++ b/fs/fuse/dev.c
> >>> @@ -1247,11 +1247,20 @@ int fuse_copy_folio(struct fuse_copy_state *cs, struct folio **foliop,
> >>>
> >>> @@ -746,6 +786,62 @@ static int fuse_uring_copy_from_ring(struct fuse_req *req,
> >>>       return err;
> >>>  }
> >>>
> >>> +static void fuse_zero_copy_release(void *priv)
> >>> +{
> >>> +     struct fuse_zero_copy_bvs *zc_bvs = priv;
> >>> +     unsigned int i;
> >>> +
> >>> +     for (i = 0; i < zc_bvs->nr_bvs; i++)
> >>> +             folio_put(page_folio(zc_bvs->bvs[i].bv_page));
> >>> +
> >>> +     kfree(zc_bvs);
> >>
> >> kvfree (see below)?
> >>
> >>> +}
> >>> +
> >>> +static int fuse_uring_set_up_zero_copy(struct fuse_ring_ent *ent,
> >>> +                                    struct fuse_req *req,
> >>> +                                    unsigned int issue_flags)
> >>> +{
> >>> +     struct fuse_args_pages *ap;
> >>> +     int err, i, ddir = 0;
> >>> +     struct fuse_zero_copy_bvs *zc_bvs;
> >>> +     struct bio_vec *bvs;
> >>> +
> >>> +     /* out_pages indicates a read, in_pages indicates a write */
> >>> +     if (req->args->out_pages)
> >>> +             ddir |= IO_BUF_DEST;
> >>> +     if (req->args->in_pages)
> >>> +             ddir |= IO_BUF_SOURCE;
> >>> +
> >>> +     ap = container_of(req->args, typeof(*ap), args);
> >>> +
> >>> +     zc_bvs = kmalloc(struct_size(zc_bvs, bvs, ap->num_folios),
> >>> +                      GFP_KERNEL_ACCOUNT);
> >>
> >> Assuming one folio per page and 1MB max_nr_pages:
> >>
> >> struct_size(zc_bvs, bvs, n) == 8 + 16 * 256 = 4104
> >>
> >> Shouldn't this be kvmalloc()?
> >
> > I'm not sure what the upper limit is for a kmalloc, but I'll change
> > this to kvmalloc().
>
>
> As far as I know kmalloc will allocate continuos pages and might fail
> under fragmentation if more than a page is needed. I'm not entirely sure
> if kvmalloc for such a small allocation will actually do the job.
> How about the attached patch? And with that maybe we should limit
> max_nr_pages to 256 for ZC?

I think kvmalloc should be fine. For size > PAGE_SIZE it tries kmalloc
first and has a fallback to vmalloc. For small allocations, kvmalloc
is just plain kmalloc.

Thanks,
Joanne