Re: [PATCH v1 2/3] io_uring/rsrc: don't rely on user vaddr alignment

[David Hildenbrand <david@redhat.com>]

On 24.06.25 12:35, Pavel Begunkov wrote:
> There is no guaranteed alignment for user pointers, however the
> calculation of an offset of the first page into a folio after
> coalescing uses some weird bit mask logic, get rid of it.
> 
> Cc: stable@vger.kernel.org
> Reported-by: David Hildenbrand <david@redhat.com>
> Fixes: a8edbb424b139 ("io_uring/rsrc: enable multi-hugepage buffer coalescing")
> Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
> ---
>   io_uring/rsrc.c | 8 +++++++-
>   io_uring/rsrc.h | 1 +
>   2 files changed, 8 insertions(+), 1 deletion(-)
> 
> diff --git a/io_uring/rsrc.c b/io_uring/rsrc.c
> index e83a294c718b..5132f8df600f 100644
> --- a/io_uring/rsrc.c
> +++ b/io_uring/rsrc.c
> @@ -734,6 +734,8 @@ bool io_check_coalesce_buffer(struct page **page_array, int nr_pages,
>   
>   	data->nr_pages_mid = folio_nr_pages(folio);
>   	data->folio_shift = folio_shift(folio);
> +	data->first_page_offset = page_array[0] - compound_head(page_array[0]);
> +	data->first_page_offset <<= PAGE_SHIFT;

Would that also cover when we have something like

nr_pages = 4
pages[0] = folio_page(folio, 1);
pages[1] = folio_page(folio, 2);
pages[2] = folio_page(folio2, 1);
pages[3] = folio_page(folio2, 2);

Note that we can create all kinds of crazy partially-mapped THP layouts 
using VMAs.

-- 
Cheers,

David / dhildenb