Re: [PATCH 1/2] mm: bail out do_swap_page() when no PTE table exist

["David Hildenbrand (Red Hat)" <david@kernel.org>]

On 12/16/25 08:59, Wei Yang wrote:
> The alloc_swap_folio() function scans the PTE table to determine the
> potential size (order) of the folio content to be swapped in.
> 
> Currently, if the call to pte_offset_map_lock() returns NULL, it
> indicates that no PTE table exists for that range. Despite this, the
> code proceeds to allocate an order-0 folio and continues the swap-in
> process. This is unnecessary if the required table is absent.
> 
> This commit modifies the logic to immediately bail out of the swap-in
> process when the PTE table is missing (i.e., pte_offset_map_lock()
> returns NULL). This ensures we do not attempt to continue swapping when
> the page table structure is incomplete or changed, preventing
> unnecessary work.
> 
> Signed-off-by: Wei Yang <richard.weiyang@gmail.com>
> Cc: Chuanhua Han <hanchuanhua@oppo.com>
> Cc: Barry Song <v-songbaohua@oppo.com>
> ---
>   mm/memory.c | 4 +++-
>   1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/memory.c b/mm/memory.c
> index 2a55edc48a65..1b8ef4f0ea60 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -4566,7 +4566,7 @@ static struct folio *alloc_swap_folio(struct vm_fault *vmf)
>   	pte = pte_offset_map_lock(vmf->vma->vm_mm, vmf->pmd,
>   				  vmf->address & PMD_MASK, &ptl);
>   	if (unlikely(!pte))
> -		goto fallback;
> +		return ERR_PTR(-EAGAIN);
>   
>   	/*
>   	 * For do_swap_page, find the highest order where the aligned range is
> @@ -4709,6 +4709,8 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
>   		    __swap_count(entry) == 1) {
>   			/* skip swapcache */
>   			folio = alloc_swap_folio(vmf);
> +			if (IS_ERR(folio))
> +				goto out;
>   			if (folio) {
>   				__folio_set_locked(folio);
>   				__folio_set_swapbacked(folio);

How would we be able to even trigger this?

Trigger a swap fault with concurrent MADV_DONTNEED and concurrent page 
table reclaim.

Is that really something we should be worrying about?

-- 
Cheers

David