Re: [v2 11/16] mm: handle PMD swap entries in non-present PMD walkers

Re: [v2 11/16] mm: handle PMD swap entries in non-present PMD walkers

[Lance Yang]

+Cc linux-mm

Please Cc linux-mm next time. Pretty clearly MM work ...

On Tue, Jun 02, 2026 at 07:24:19AM -0700, Usama Arif wrote:
[...]
>diff --git a/mm/mincore.c b/mm/mincore.c
>index e5d13eea9234..3fee8a7b9d9d 100644
>--- a/mm/mincore.c
>+++ b/mm/mincore.c
>@@ -172,7 +172,19 @@ static int mincore_pte_range(pmd_t *pmd, unsigned long addr, unsigned long end,
> 
> 	ptl = pmd_trans_huge_lock(pmd, vma);
> 	if (ptl) {
>-		memset(vec, 1, nr);
>+		if (pmd_present(*pmd)) {
>+			memset(vec, 1, nr);
>+		} else {
>+			/*
>+			 * Non-present PMD: migration, device-private, or PMD
>+			 * swap entry. Route through mincore_swap() the same way
>+			 * the PTE path does -- the swap entry covers all 512
>+			 * slots, so the whole vec gets the same answer.
>+			 */
>+			softleaf_t entry = softleaf_from_pmd(*pmd);
>+
>+			memset(vec, mincore_swap(entry, false), nr);

Looks buggy ...

That assumes one swap-cache lookup is enough for whole PMD-sized range.
I don't think that always holds ...

See do_huge_pmd_swap_page():

---8<---
	folio = swap_cache_get_folio(swp_entry);
[...]
	/*
	 * Folio should be PMD-sized; if not (e.g. split in swap cache),
	 * split the PMD swap entry and retry at PTE level.
	 */
	if (folio_nr_pages(folio) != HPAGE_PMD_NR) {
		folio_unlock(folio);
		folio_put(folio);
		goto split_fallback;
	}
---

it handles the case where swap_cache_get_folio() returns a folio that
is no longer PMD-sized. E.g. because it was split in the swap cache
while the PMD swap entry was installed. Then it split the PMD swap entry
and retries at PTE level :)

unuse_pmd_entry() has the same fallback. Can mincore hit that case?

Maybe the comment right above should say something like:

"
One lookup is enough for a PMD-sized swapcache folio. If the swapcache
was split, check the per-page swap slots.
"

Hopefully, I'm not missing something here :D

Cheers, Lance