Re: [PATCH] userfaultfd: change src_folio after ensuring it's unpinned in UFFDIO_MOVE

[David Hildenbrand <david@redhat.com>]

On 04.04.24 22:07, Suren Baghdasaryan wrote:
> On Thu, Apr 4, 2024 at 10:21 AM Matthew Wilcox <willy@infradead.org> wrote:
>>
>> On Thu, Apr 04, 2024 at 10:17:26AM -0700, Lokesh Gidra wrote:
>>> -             folio_move_anon_rmap(src_folio, dst_vma);
>>> -             WRITE_ONCE(src_folio->index, linear_page_index(dst_vma, dst_addr));
>>> -
>>>                src_pmdval = pmdp_huge_clear_flush(src_vma, src_addr, src_pmd);
>>>                /* Folio got pinned from under us. Put it back and fail the move. */
>>>                if (folio_maybe_dma_pinned(src_folio)) {
>>> @@ -2270,6 +2267,9 @@ int move_pages_huge_pmd(struct mm_struct *mm, pmd_t *dst_pmd, pmd_t *src_pmd, pm
>>>                        goto unlock_ptls;
>>>                }
>>>
>>> +             folio_move_anon_rmap(src_folio, dst_vma);
>>> +             WRITE_ONCE(src_folio->index, linear_page_index(dst_vma, dst_addr));
>>> +
>>
>> This use of WRITE_ONCE scares me.  We hold the folio locked.  Why do
>> we need to use WRITE_ONCE?  Who's looking at folio->index without
>> holding the folio lock?
> 
> Indeed that seems to be unnecessary here. Both here and in
> move_present_pte() we are holding folio lock while moving the page. I
> must have just blindly copied that from Andrea's original patch [1].

Agreed, I don't think it is required for ->index. (I also don't spot any 
corresponding READ_ONCE)

-- 
Cheers,

David / dhildenb