Re: [PATCH 2/2] mm/rmap: Improve mlock tracking for large folios

Lorenzo Stoakes
2025-09-18T13:10:44Z

On Thu, Sep 18, 2025 at 12:21:57PM +0100, kirill@shutemov.name wrote:
> From: Kiryl Shutsemau <kas@kernel.org>
>
> The kernel currently does not mlock large folios when adding them to
> rmap, stating that it is difficult to confirm that the folio is fully
> mapped and safe to mlock it. However, nowadays the caller passes a
> number of pages of the folio that are getting mapped, making it easy to
> check if the entire folio is mapped to the VMA.
>
> mlock the folio on rmap if it is fully mapped to the VMA.
>
> Signed-off-by: Kiryl Shutsemau <kas@kernel.org>

The logic looks good to me, so:

Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>

But note the comments below.

> ---
> mm/rmap.c | 13 ++++---------
> 1 file changed, 4 insertions(+), 9 deletions(-)
>
> diff --git a/mm/rmap.c b/mm/rmap.c
> index 568198e9efc2..ca8d4ef42c2d 100644
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -1478,13 +1478,8 @@ static __always_inline void __folio_add_anon_rmap(struct folio *folio,
> PageAnonExclusive(cur_page), folio);
> }
>
> - /*
> - * For large folio, only mlock it if it's fully mapped to VMA. It's
> - * not easy to check whether the large folio is fully mapped to VMA
> - * here. Only mlock normal 4K folio and leave page reclaim to handle
> - * large folio.
> - */
> - if (!folio_test_large(folio))
> + /* Only mlock it if the folio is fully mapped to the VMA */
> + if (folio_nr_pages(folio) == nr_pages)

OK this is nice, as partially mapped will have folio_nr_pages() != nr_pages. So
logically this must be correct.

> mlock_vma_folio(folio, vma);
> }
>
> @@ -1620,8 +1615,8 @@ static __always_inline void __folio_add_file_rmap(struct folio *folio,
> nr = __folio_add_rmap(folio, page, nr_pages, vma, level, &nr_pmdmapped);
> __folio_mod_stat(folio, nr, nr_pmdmapped);
>
> - /* See comments in folio_add_anon_rmap_*() */
> - if (!folio_test_large(folio))
> + /* Only mlock it if the folio is fully mapped to the VMA */
> + if (folio_nr_pages(folio) == nr_pages)
> mlock_vma_folio(folio, vma);
> }
>
> --
> 2.50.1
>

I see in try_to_unmap_one():

if (!(flags & TTU_IGNORE_MLOCK) &&
(vma->vm_flags & VM_LOCKED)) {
/* Restore the mlock which got missed */
if (!folio_test_large(folio))
mlock_vma_folio(folio, vma);

Do we care about this?

It seems like folio_referenced_one() does some similar logic:

if (vma->vm_flags & VM_LOCKED) {
if (!folio_test_large(folio) || !pvmw.pte) {
/* Restore the mlock which got missed */
mlock_vma_folio(folio, vma);
page_vma_mapped_walk_done(&pvmw);
pra->vm_flags |= VM_LOCKED;
return false; /* To break the loop */
}

...

if ((vma->vm_flags & VM_LOCKED) &&
folio_test_large(folio) &&
folio_within_vma(folio, vma)) {
unsigned long s_align, e_align;

s_align = ALIGN_DOWN(start, PMD_SIZE);
e_align = ALIGN_DOWN(start + folio_size(folio) - 1, PMD_SIZE);

/* folio doesn't cross page table boundary and fully mapped */
if ((s_align == e_align) && (ptes == folio_nr_pages(folio))) {
/* Restore the mlock which got missed */
mlock_vma_folio(folio, vma);
pra->vm_flags |= VM_LOCKED;
return false; /* To break the loop */
}
}

So maybe we could do something similar in try_to_unmap_one()?