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

[Usama Arif <usama.arif@linux.dev>]

On 19/08/2026 06:38, Lance Yang wrote:
> +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?


Good catch!

I am going to introduce a helper like below and use it in the next
revision. Thanks!

static inline bool folio_has_hwpoisoned_subpage(struct folio *folio)
{
	unsigned long i;

	for (i = 0; i < folio_nr_pages(folio); i++)
		if (PageHWPoison(folio_page(folio, i)))
			return true;
	return false;
}

> 
>> +
>> +	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