Re: [PATCH 2/2] mm: avoid unnecessary PTE table lock during initial swap folio scan

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

On 12/16/25 08:59, Wei Yang wrote:
> The alloc_swap_folio() function performs an initial scan of the PTE
> table solely to determine the potential size (order) of the folio
> content that needs to be swapped in.
> 
> Locking the PTE table during this initial read is unnecessary for two
> reasons:
> 
>    * We are not writing to the PTE table at this stage.
> 
>    * The code will re-check and lock the table again immediately before
>      any actual modification is attempted.
> 
> This commit refactors the initial scan to map the PTE table without
> acquiring the lock. This reduces contention and overhead, improving
> performance of the swap-in path.
> 
> 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 | 6 ++----
>   1 file changed, 2 insertions(+), 4 deletions(-)
> 
> diff --git a/mm/memory.c b/mm/memory.c
> index 1b8ef4f0ea60..f8d6adfa83d7 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -4529,7 +4529,6 @@ static struct folio *alloc_swap_folio(struct vm_fault *vmf)
>   	struct folio *folio;
>   	unsigned long addr;
>   	softleaf_t entry;
> -	spinlock_t *ptl;
>   	pte_t *pte;
>   	gfp_t gfp;
>   	int order;
> @@ -4563,8 +4562,7 @@ static struct folio *alloc_swap_folio(struct vm_fault *vmf)
>   	if (!orders)
>   		goto fallback;
>   
> -	pte = pte_offset_map_lock(vmf->vma->vm_mm, vmf->pmd,
> -				  vmf->address & PMD_MASK, &ptl);
> +	pte = pte_offset_map(vmf->pmd, vmf->address & PMD_MASK);
>   	if (unlikely(!pte))

Can can_swapin_thp() deal with concurrent unmap and possible freeing of 
pages+swap?

We have some code that depends on swap entries stabilizing the swap 
device etc; the moment you allow for that concurrently to go away you 
open a can of worns.

-- 
Cheers

David