Re: [PATCH 7.2.y] mm/damon/vaddr: drop last same folio access check optimization

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:

Pre-existing issues:
- [High] DAMOS PTE-level PMD iteration incorrectly advances past unrelated memory when encountering large folio tail pages.
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commit d786b1179a857b410a9db73cdc4ee0d7bdcbede5
Author: SJ Park <sj@kernel.org>

mm/damon/vaddr: drop last same folio access check optimization
    
This commit drops the last same folio access check optimization in DAMON.
The optimization can race when multiple kdamonds are running, and its
impact is doubtful.

Link: https://lore.kernel.org/20260715031002.108504-4-sj@kernel.org
Link: https://lore.kernel.org/20260621204050.10993-1-sj@kernel.org [1]

> diff --git a/mm/damon/vaddr.c b/mm/damon/vaddr.c
> index 2c1c1952c008d..404785128ffd7 100644
> --- a/mm/damon/vaddr.c
> +++ b/mm/damon/vaddr.c
[ ... ]

[Severity: High]
This is a pre-existing issue, but does this code incorrectly advance past
unrelated memory when encountering large folio tail pages?

Looking at damos_va_migrate_pmd_entry() and damos_va_stat_pmd_entry() in
mm/damon/vaddr.c, the PTE iterator unconditionally advances by the full folio
size:

mm/damon/vaddr.c:damos_va_migrate_pmd_entry() {
    ...
	for (; addr < next; pte += nr, addr += nr * PAGE_SIZE) {
		nr = 1;
		ptent = ptep_get(pte);

		if (pte_none(ptent) || !pte_present(ptent))
			continue;
		folio = vm_normal_folio(walk->vma, addr, ptent);
    ...
		damos_va_migrate_dests_add(folio, walk->vma, addr, dests,
				migration_lists);
		nr = folio_nr_pages(folio);
	}
    ...
}

If the loop encounters a PTE pointing to a tail page of a large folio,
folio_nr_pages(folio) returns the size of the entire large folio. Because the
current PTE does not point to the head of the folio, advancing pte and
addr by the full folio size will overshoot into unrelated page table
entries, skipping memory regions during migration and stats scanning.

Could this lead to incorrect functional behavior where unrelated memory
regions are silently skipped by DAMOS actions?

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260909055319.263738-1-sj@kernel.org?part=1