Re: [PATCH] lib/scatterlist: fix sg_page_count and sg_dma_page_count

[Jason Gunthorpe <jgg@ziepe.ca>]

On Tue, Mar 10, 2026 at 05:49:23AM +0000, Kasireddy, Vivek wrote:
> There are a couple of reasons why we got rid of the pages array:
> - Back then, there was some confusion about whether a struct page would
>   exist or not for tail pages when HVO is enabled. Regardless, there was also
>   a concern about exposing tail pages outside hugetlb code.

The existing code relies on struct page for the vmap:

        for (pg = 0; pg < ubuf->pagecount; pg++)
                pages[pg] = folio_page(ubuf->folios[pg],
                                       ubuf->offsets[pg] >> PAGE_SHIFT);

Tail pages always exist, they are required by many interfaces.

> - And, we also wanted to prepare for a future where struct page would not
>   exist anymore, so, it made sense to just use folios only.

If you can 100% stick with whole folios then great, but we don't have
the APIs for that cases udmabuf needs right now. Most likely we'd
expect to use phys_addr_t for scatterlist and direct full folio for
vmap. Neither is helped by the datastructure in udmabuf.

Jason