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

[David Hildenbrand <david@redhat.com>]

On 24.06.25 14:20, 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;

Ah okay, that makes sense.

It might be clearer at some point to coalesce folio ranges (e.g., 
folio,idx,len) instead, representing them in a different temporary 
structure.

-- 
Cheers,

David / dhildenb