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++;
> }
