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

[Zhang Yi]

Gentle ping – could anyone take this patch?

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;