Re: [PATCH v6 06/12] mm: swap in PMD swap entries as whole THPs during swapoff

[Lance Yang <lance.yang@linux.dev>]

+Cc Miaohe

On Tue, Aug 18, 2026 at 06:09:47AM -0700, Usama Arif wrote:
[...]
>+#ifdef CONFIG_THP_SWAP
>+/*
>+ * unuse_pmd - Map a locked folio at PMD granularity during swapoff.
>+ *
>+ * The caller provides a locked, swapped-in folio.  Returns 0 on success
>+ * (PMD was mapped).  Returns -EAGAIN if the swap cache folio no longer
>+ * matches the entry or the PMD changed under the lock (try_to_unuse will
>+ * rescan). Returns -EIO if the folio is not uptodate or contains a poisoned
>+ * subpage; in that case the PMD is split so unuse_pte_range() can handle
>+ * individual pages.
>+ */
>+static int unuse_pmd(struct vm_area_struct *vma, pmd_t *pmd,
>+		     unsigned long addr, softleaf_t entry,
>+		     struct folio *folio)
>+{
>+	struct mm_struct *mm = vma->vm_mm;
>+	struct page *page;
>+	pmd_t new_pmd, old_pmd;
>+	spinlock_t *ptl;
>+	rmap_t rmap_flags = RMAP_NONE;
>+	bool exclusive;
>+
>+	if (unlikely(!folio_matches_swap_entry(folio, entry)))
>+		return -EAGAIN;
>+
>+	if (unlikely(!folio_test_uptodate(folio))) {
>+		/* Let PTE fallback reread each slot independently. */
>+		swap_cache_del_folio(folio);
>+		__split_huge_pmd(vma, pmd, addr, false);
>+		return -EIO;
>+	}
>+
>+	if (unlikely(folio_contain_hwpoisoned_page(folio))) {
>+		/* Let PTE fallback isolate the poisoned subpages. */
>+		__split_huge_pmd(vma, pmd, addr, false);
>+		return -EIO;
>+	}

Hmm ... can this miss a poisoned tail?

memory_failure() sets PageHWPoison(p) before taking folio_lock(), but
PG_has_hwpoisoned is only set later. Since unuse_pmd_entry() holds folio
lock across this check, memory_failure() can be blocked on folio_lock()
with a tail already poisoned, IIUC ...

folio_contain_hwpoisoned_page() then sees neither a poisoned head nor
PG_has_hwpoisoned, and set_pmd_at() maps that tail through a normal PMD.

Note that unuse_pte() checks PageHWPoison(page) directly. Should we check
each subpage here as well before installing the PMD?

>+
>+	page = folio_page(folio, 0);
>+
>+	ptl = pmd_lock(mm, pmd);
>+	old_pmd = pmdp_get(pmd);
>+
>+	if (!pmd_is_swap_entry(old_pmd) ||
>+	    softleaf_from_pmd(old_pmd).val != entry.val) {
>+		spin_unlock(ptl);
>+		return -EAGAIN;
>+	}
>+
>+	exclusive = pmd_swp_exclusive(old_pmd);
>+
>+	/*
>+	 * Some architectures may have to restore extra metadata to the folio
>+	 * when reading from swap. This metadata may be indexed by swap entry
>+	 * so this must be called before folio_put_swap().
>+	 */
>+	arch_swap_restore(folio_swap(entry, folio), folio);
>+
>+	add_mm_counter(mm, MM_ANONPAGES, HPAGE_PMD_NR);
>+	add_mm_counter(mm, MM_SWAPENTS, -HPAGE_PMD_NR);
>+
>+	new_pmd = folio_mk_pmd(folio, vma->vm_page_prot);
>+	new_pmd = pmd_mkold(new_pmd);
>+	if (pmd_swp_soft_dirty(old_pmd))
>+		new_pmd = pmd_mksoft_dirty(new_pmd);
>+	if (pmd_swp_uffd(old_pmd))
>+		new_pmd = pmd_mkuffd(new_pmd);
>+	if (pmd_swp_uffd(old_pmd) && userfaultfd_rwp(vma))
>+		new_pmd = pmd_modify(new_pmd, PAGE_NONE);
>+
>+	if (exclusive)
>+		rmap_flags |= RMAP_EXCLUSIVE;
>+
>+	folio_get(folio);
>+	if (!folio_test_anon(folio))
>+		folio_add_new_anon_rmap(folio, vma, addr, rmap_flags);
>+	else
>+		folio_add_anon_rmap_pmd(folio, page, vma, addr, rmap_flags);
>+
>+	set_pmd_at(mm, addr, pmd, new_pmd);
>+	folio_put_swap(folio, NULL);
>+
>+	spin_unlock(ptl);
>+
>+	folio_free_swap(folio);
>+	return 0;
>+}

[...]

Cheers, Lance