Re: [PATCH v1] mm: multi-gen LRU: reduce repeated scans in rmap

[Yu Zhao]

On Sun, Jan 15, 2023 at 5:57 AM Jinyu Tang <tjytimi@xxxxxxx> wrote:
>
> The folio is physically and virtually contiguous. If a folio have
> more than one pages, lru_gen_look_around() will run several times in
> the while loop in folio_referenced_one(), but most of times is
> unnecessary. Because these pages always belong to the same pmd and
> vma, lru_gen_look_around() will scan the same range.

Thanks -- the commit message is quite clear, so I think I understand
what you're thinking.

Let me clarify:
1. First of all, there are no repeated scans, because after
lru_gen_look_around() clears the A-bit in a range, the pte_young()
test stops it from going into the same range again.
2. Of course, pte_young() can become true later, but this is not a
problem because it's cache hot.
3. Physically and virtually contiguous mapping existed before folios:
a THP can be mapped by 512 PTEs.

>         while (page_vma_mapped_walk(&pvmw)) {
>                 address = pvmw.address;
> @@ -825,7 +826,13 @@ static bool folio_referenced_one(struct folio *folio,
>                 if (pvmw.pte) {
>                         if (lru_gen_enabled() && pte_young(*pvmw.pte) &&
>                             !(vma->vm_flags & (VM_SEQ_READ | VM_RAND_READ))) {
> -                               lru_gen_look_around(&pvmw);
> +                               unsigned long pmd_now = pvmw.address & PMD_MASK;
> +                               unsigned long pmd_former = former_address & PMD_MASK;
> +
> +                               if ((!former_address) || (pmd_now != pmd_former)) {
> +                                       lru_gen_look_around(&pvmw);
> +                                       former_address = pvmw.address;
> +                               }
>                                 referenced++;
>                         }