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

["Christian König" <christian.koenig@amd.com>]

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.

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

Regards,
Christian.

> 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
>