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

[Pavel Begunkov <asml.silence@gmail.com>]

On 6/24/25 13:20, Pavel Begunkov wrote:
> On 6/24/25 12:53, David Hildenbrand wrote:
>> 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.
> 
> It'll see that pages[2] is not the first page of folio2
> and return that it can't be coalesced
> 
> if (/* ... */ || folio_page_idx(folio, page_array[i]) != 0)
>      return false;

To elaborate, we're only coalescing if for all but the first resulting
bvec segment starts from the beginning of its folio, and all but the
last bvec segment ends at the right border of the folio. IOW, all
middle bvecs should fully cover their folios, and the first and the
last bvecs should align by the right and left borders of their folios
correspondingly.

-- 
Pavel Begunkov