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 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;

-- 
Pavel Begunkov