Re: [PATCH net-next 2/4] udmabuf: emit one sg entry per pinned folio

[Bobby Eshleman <bobbyeshleman@gmail.com>]

On Fri, Jun 05, 2026 at 11:30:07AM +0200, Christian König wrote:
> On 6/4/26 02:42, Bobby Eshleman wrote:
> > From: Bobby Eshleman <bobbyeshleman@meta.com>
> > 
> > get_sg_table() emitted one PAGE_SIZE sg entry per page even when the
> > underlying folio was larger.
> > 
> > Instead, walk folios[] and emit one sg entry per folio. When folios
> > represent large pages (as is for MFD_HUGETLB), each sg entry is a large
> > page. Normal PAGE_SIZE sg tables are unchanged.
> > 
> > Required by net/core/devmem to support rx-buf-size > PAGE_SIZE with
> > udmabuf.
> 
> That doesn't explain why this is required.

Sure, can definitely add. Devmem currently requires dmabuf sg entries to
be length and size aligned when it allocates niovs for NIC page pools.
Though udmabuf is not violating any dmabuf contract by emitting
PAGE_SIZE entries and the above restriction is probably more a
shortfalling of devmem, by emitting a single entry per folio this patch
allows udmabuf to be used by devmem for large pages.

> 
> Please note that accessing the pages/folio of an sg-table returned by DMA-buf is illegal and strictly forbidden!
> 
> Regards,
> Christian.

It seems both devmem and io_uring zcrx at least introspect through to
the sg-table to build NIC page pools (not accessing the memory itself,
however). Is there a better way?

Best,
Bobby

> 
> > Signed-off-by: Bobby Eshleman <bobbyeshleman@meta.com>
> > ---
> >  drivers/dma-buf/udmabuf.c | 47 ++++++++++++++++++++++++++++++++++++++++++-----
> >  1 file changed, 42 insertions(+), 5 deletions(-)
> > 
> > diff --git a/drivers/dma-buf/udmabuf.c b/drivers/dma-buf/udmabuf.c
> > index 94b8ecb892bb..f28dd3788ada 100644
> > --- a/drivers/dma-buf/udmabuf.c
> > +++ b/drivers/dma-buf/udmabuf.c
> > @@ -141,26 +141,63 @@ static void vunmap_udmabuf(struct dma_buf *buf, struct iosys_map *map)
> >         vm_unmap_ram(map->vaddr, ubuf->pagecount);
> >  }
> > 
> > +/* Return the number of contiguous pages backed by the folio at @i.
> > + * A udmabuf may map only part of a folio, or reference the same folio
> > + * in multiple non-contiguous runs, so folio_nr_pages() can't be used.
> > + */
> > +static pgoff_t udmabuf_folio_nr_pages(struct udmabuf *ubuf, pgoff_t i)
> > +{
> > +       struct folio *f = ubuf->folios[i];
> > +       pgoff_t j;
> > +
> > +       for (j = 1; i + j < ubuf->pagecount; j++) {
> > +               if (ubuf->folios[i + j] != f)
> > +                       break;
> > +               /* Same folio, but not a sequential offset within it. */
> > +               if (ubuf->offsets[i + j] != ubuf->offsets[i] + j * PAGE_SIZE)
> > +                       break;
> > +       }
> > +       return j;
> > +}
> > +
> > +/* Count the contiguous folio runs in @ubuf, one sg entry per run. */
> > +static unsigned int udmabuf_sg_nents(struct udmabuf *ubuf)
> > +{
> > +       unsigned int nents = 0;
> > +       pgoff_t i;
> > +
> > +       for (i = 0; i < ubuf->pagecount; i += udmabuf_folio_nr_pages(ubuf, i))
> > +               nents++;
> > +       return nents;
> > +}
> > +
> >  static struct sg_table *get_sg_table(struct device *dev, struct dma_buf *buf,
> >                                      enum dma_data_direction direction)
> >  {
> >         struct udmabuf *ubuf = buf->priv;
> > -       struct sg_table *sg;
> >         struct scatterlist *sgl;
> > -       unsigned int i = 0;
> > +       struct sg_table *sg;
> > +       pgoff_t i, run;
> > +       unsigned int nents;
> >         int ret;
> > 
> > +       nents = udmabuf_sg_nents(ubuf);
> > +
> >         sg = kzalloc_obj(*sg);
> >         if (!sg)
> >                 return ERR_PTR(-ENOMEM);
> > 
> > -       ret = sg_alloc_table(sg, ubuf->pagecount, GFP_KERNEL);
> > +       ret = sg_alloc_table(sg, nents, GFP_KERNEL);
> >         if (ret < 0)
> >                 goto err_alloc;
> > 
> > -       for_each_sg(sg->sgl, sgl, ubuf->pagecount, i)
> > -               sg_set_folio(sgl, ubuf->folios[i], PAGE_SIZE,
> > +       sgl = sg->sgl;
> > +       for (i = 0; i < ubuf->pagecount; i += run) {
> > +               run = udmabuf_folio_nr_pages(ubuf, i);
> > +               sg_set_folio(sgl, ubuf->folios[i], run << PAGE_SHIFT,
> >                              ubuf->offsets[i]);
> > +               sgl = sg_next(sgl);
> > +       }
> > 
> >         ret = dma_map_sgtable(dev, sg, direction, 0);
> >         if (ret < 0)
> > 
> > --
> > 2.53.0-Meta
> > 
>