Re: [PATCH v3 2/4] mm: drop stale folio_ref_count()==1 check in do_swap_page reuse logic

[David Hildenbrand (Arm)]

On 7/2/26 01:59, Barry Song (Xiaomi) wrote:
> The "we just allocated them without exposing them to the swapcache"
> case no longer exists, as Kairui has routed synchronous I/O through
> the swapcache as well in his series "unify swapin use swap cache and
> cleanup flags"[1]. As a result, folio_ref_count() should never be 1
> in this path, since at least two references are held (base ref plus
> swapcache). Remove the folio_ref_count()==1 check and update the
> comment accordingly.

Sashiko points out two minor things (one flagged as medium, lol, sure sure).

Here, you can clarify that folio_ref_count()==1 is true for freshly allocated
pages (due to the KSM check) in which case exclusive=true already.

> 
> [1] https://lore.kernel.org/all/20251220-swap-table-p2-v5-0-8862a265a033@xxxxxxxxxxx/
> 
> Acked-by: Usama Arif <usama.arif@xxxxxxxxx>
> Reviewed-by: Kairui Song <kasong@xxxxxxxxxxx>
> Reviewed-by: Baoquan He <baoquan.he@xxxxxxxxx>
> Acked-by: Shakeel Butt <shakeel.butt@xxxxxxxxx>
> Signed-off-by: Barry Song (Xiaomi) <baohua@xxxxxxxxxx>
> ---
>  mm/memory.c | 9 +++------
>  1 file changed, 3 insertions(+), 6 deletions(-)
> 
> diff --git a/mm/memory.c b/mm/memory.c
> index 87da78eb1abd..71e9d394816b 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -5046,13 +5046,10 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
>  		pte = pte_mkuffd_wp(pte);
>  
>  	/*
> -	 * Same logic as in do_wp_page(); however, optimize for pages that are
> -	 * certainly not shared either because we just allocated them without
> -	 * exposing them to the swapcache or because the swap entry indicates
> -	 * exclusivity.
> +	 * Similar logic as in do_wp_page(); however, optimize for pages that are
> +	 * certainly not because the swap entry indicates exclusivity.

s/not/not shared/

but likely you should just simplify to "... pages that are certainly exclusive."

>  	 */
> -	if (!folio_test_ksm(folio) &&
> -	    (exclusive || folio_ref_count(folio) == 1)) {
> +	if (exclusive) {
>  		if ((vma->vm_flags & VM_WRITE) && !userfaultfd_pte_wp(vma, pte) &&
>  		    !pte_needs_soft_dirty_wp(vma, pte)) {
>  			pte = pte_mkwrite(pte, vma);


-- 
Cheers,

David