Re: [PATCH v2 3/9] mm/swap: avoid doing extra unlock error checks for direct swapin

[Huang, Ying]

Kairui Song <ryncsn@xxxxxxxxx> writes:

> From: Kairui Song <kasong@xxxxxxxxxxx>
>
> When swapping in a page, mem_cgroup_swapin_charge_folio is called for
> new allocated folio, nothing else is referencing the folio so no need
> to set the lock bit early. This avoided doing extra unlock checks
> on the error path.
>
> Signed-off-by: Kairui Song <kasong@xxxxxxxxxxx>
> ---
>  mm/swap_state.c | 13 ++++++-------
>  1 file changed, 6 insertions(+), 7 deletions(-)
>
> diff --git a/mm/swap_state.c b/mm/swap_state.c
> index 24cb93ed5081..6130de8d5226 100644
> --- a/mm/swap_state.c
> +++ b/mm/swap_state.c
> @@ -881,16 +881,15 @@ struct folio *swapin_direct(swp_entry_t entry, gfp_t gfp_mask,
>  	folio = vma_alloc_folio(GFP_HIGHUSER_MOVABLE, 0,
>  				vma, vmf->address, false);
>  	if (folio) {
> -		__folio_set_locked(folio);
> -		__folio_set_swapbacked(folio);
> -
> -		if (mem_cgroup_swapin_charge_folio(folio,
> -					vma->vm_mm, GFP_KERNEL,
> -					entry)) {
> -			folio_unlock(folio);
> +		if (mem_cgroup_swapin_charge_folio(folio, vma->vm_mm,
> +						   GFP_KERNEL, entry)) {
>  			folio_put(folio);
>  			return NULL;
>  		}
> +
> +		__folio_set_locked(folio);
> +		__folio_set_swapbacked(folio);
> +
>  		mem_cgroup_swapin_uncharge_swap(entry);
>  
>  		shadow = get_shadow_from_swap_cache(entry);

I don't find any issue with the patch.  But another caller of
mem_cgroup_swapin_charge_folio() in __read_swap_cache_async() setups
newly allocated folio in the same way before the change.  Better to keep
them same?  Because the benefit of change is small too.

--
Best Regards,
Huang, Ying