Re: [PATCH v2] mm: do not install PMD mappings when handling a COW fault

[Zhang Yi]

On 5/19/2026 5:02 PM, William Kucharski wrote:
> 
> 
>> On May 19, 2026, at 02:36, Zhang Yi <yi.zhang@xxxxxxxxxxxxxxx> wrote:
>>
>> Gentle ping – could anyone take this patch?
>>
>> Thanks,
>> Yi.
> 
> Could you update the comment to clarify why you shouldn't install PMD mappings
> while doing CoW rather than just state it should never be done?

OK, will do.

Thanks,
Yi.

> 
>>
>> On 10/24/2025 6:22 PM, Zhang Yi wrote:
>>> From: Zhang Yi <yi.zhang@xxxxxxxxxx>
>>>
>>> When pinning a page with FOLL_LONGTERM in a CoW VMA and a PMD-aligned
>>> (2MB on x86) large folio follow_page_mask() failed to obtain a valid
>>> anonymous page, resulting in an infinite loop issue. The specific
>>> triggering process is as follows:
>>>
>>> 1. User call mmap with a 2MB size in MAP_PRIVATE mode for a file that
>>>   has a 2MB large folio installed in the page cache.
>>>
>>>   addr = mmap(NULL, 2*1024*1024, PROT_READ, MAP_PRIVATE, file_fd, 0);
>>>
>>> 2. The kernel driver pass this mapped address to pin_user_pages_fast()
>>>   in FOLL_LONGTERM mode.
>>>
>>>   pin_user_pages_fast(addr, 512, FOLL_LONGTERM, pages);
>>>
>>>  ->  pin_user_pages_fast()
>>>  |   gup_fast_fallback()
>>>  |    __gup_longterm_locked()
>>>  |     __get_user_pages_locked()
>>>  |      __get_user_pages()
>>>  |       follow_page_mask()
>>>  |        follow_p4d_mask()
>>>  |         follow_pud_mask()
>>>  |          follow_pmd_mask() //pmd_leaf(pmdval) is true because the
>>>  |                            //huge PMD is installed. This is normal
>>>  |                            //in the first round, but it shouldn't
>>>  |                            //happen in the second round.
>>>  |           follow_huge_pmd() //require an anonymous page
>>>  |            return -EMLINK;
>>>  |   faultin_page()
>>>  |    handle_mm_fault()
>>>  |     wp_huge_pmd() //remove PMD and fall back to PTE
>>>  |     handle_pte_fault()
>>>  |      do_pte_missing()
>>>  |       do_fault()
>>>  |        do_read_fault() //FAULT_FLAG_WRITE is not set
>>>  |         finish_fault()
>>>  |          do_set_pmd() //install a huge PMD again, this is wrong!!!
>>>  |      do_wp_page() //create private anonymous pages
>>>  <-    goto retry;
>>>
>>> Due to an incorrectly large PMD set in do_read_fault(),
>>> follow_pmd_mask() always returns -EMLINK, causing an infinite loop.
>>>
>>> David pointed out that we can preallocate a page table and remap the PMD
>>> to be mapped by a PTE table in wp_huge_pmd() in the future. But now we
>>> can avoid this issue by not installing PMD mappings when handling a COW
>>> and unshare fault in do_set_pmd().
>>>
>>> Fixes: a7f226604170 ("mm/gup: trigger FAULT_FLAG_UNSHARE when R/O-pinning a possibly shared anonymous page")
>>> Reported-by: Karol Wachowski <karol.wachowski@xxxxxxxxxxxxxxx>
>>> Closes: https://lore.kernel.org/linux-ext4/844e5cd4-462e-4b88-b3b5-816465a3b7e3@xxxxxxxxxxxxxxx/
>>> Suggested-by: David Hildenbrand <david@xxxxxxxxxx>
>>> Signed-off-by: Zhang Yi <yi.zhang@xxxxxxxxxx>
>>> Acked-by: David Hildenbrand <david@xxxxxxxxxx>
>>> ---
>>> mm/memory.c | 5 +++++
>>> 1 file changed, 5 insertions(+)
>>>
>>> diff --git a/mm/memory.c b/mm/memory.c
>>> index 0ba4f6b71847..0748a31367df 100644
>>> --- a/mm/memory.c
>>> +++ b/mm/memory.c
>>> @@ -5212,6 +5212,11 @@ vm_fault_t do_set_pmd(struct vm_fault *vmf, struct folio *folio, struct page *pa
>>> if (!thp_vma_suitable_order(vma, haddr, PMD_ORDER))
>>> return ret;
>>>
>>> + /* We're about to trigger CoW, so never map it through a PMD. */
>>> + if (is_cow_mapping(vma->vm_flags) &&
>>> +    (vmf->flags & (FAULT_FLAG_WRITE|FAULT_FLAG_UNSHARE)))
>>> + return ret;
>>> +
>>> if (folio_order(folio) != HPAGE_PMD_ORDER)
>>> return ret;
>>> page = &folio->page;
>>
>>