[PATCH v5 23/38] mm/mlock: eliminate weird VMA_IO_BIT abuse and simplify
David Hildenbrand (Arm)
david at kernel.org
Mon Oct 12 11:23:15 PDT 2026
On 10/9/26 20:04, Lorenzo Stoakes (ARM) wrote:
> When performing mlock() or munlock() otherwise normal VMAs have VMA_IO_BIT
> solely to fix a race with migration which might otherwise double-count
> mlock VMAs.
>
> This is unnecessary - at the point of applying folio mlock state, whether
> setting or clearing PG_mlocked, we know whether or not we are locking.
>
> Solve this in two ways - thread a boolean through the page table walk
> indicating whether a lock or unlock is being performed, and run a locking
> walk with VMA_LOCKONFAULT_BIT set and VMA_LOCKED_BIT cleared.
>
> This state never occurs otherwise, as VMA_LOCKONFAULT_BIT always implies
> VMA_LOCKED_BIT. These are also always cleared together.
>
> Then, update folio_add_lru_vma() and mlock_folio() to check only for
> VMA_LOCKED_BIT, and update try_to_unmap_one() to check for VMA_LOCKED_MASK
> instead.
>
> Also remove the useless invocation of allow_mlock_munlock() which simply
> returns true if unlocking and instead rename it to allow_mlock() and only
> call it when locking.
>
> With the other mlock abuse of VMA_IO_BIT addressed, update
> mlock_vma_folio() and folio_add_lru_vma() to simply test for
> VMA_LOCKED_BIT.
>
> The munlock_vma_folio() function tests VMA_LOCKED_MASK instead, as an
> unmap racing with the locking walk must still munlock folios the walk has
> already counted.
>
> Additionally, in order to ensure a racing unmap/migration releases the
> reference munlock_folio() took (whose put was batched), check
> VMA_LOCKED_MASK before calling mlock_drain_local() in try_to_unmap_one()
> and try_to_migrate_one().
>
> While here, also replace some deprecated VMA flag predicates.
>
> Reviewed-by: Zi Yan <ziy at nvidia.com>
> Signed-off-by: Lorenzo Stoakes (ARM) <ljs at kernel.org>
> ---
> mm/folio.c | 2 +-
> mm/internal.h | 10 +++++++---
> mm/mlock.c | 51 +++++++++++++++++++--------------------------------
> mm/rmap.c | 8 +++++---
> 4 files changed, 32 insertions(+), 39 deletions(-)
>
> diff --git a/mm/folio.c b/mm/folio.c
> index 47a437e0f7fd..35e242b48870 100644
> --- a/mm/folio.c
> +++ b/mm/folio.c
> @@ -505,7 +505,7 @@ void folio_add_lru_vma(struct folio *folio, struct vm_area_struct *vma)
> {
> VM_BUG_ON_FOLIO(folio_test_lru(folio), folio);
>
> - if (unlikely((vma->vm_flags & (VM_LOCKED | VM_SPECIAL)) == VM_LOCKED))
> + if (vma_test(vma, VMA_LOCKED_BIT))
> mlock_new_folio(folio);
> else
> folio_add_lru(folio);
> diff --git a/mm/internal.h b/mm/internal.h
> index 4ae08fedffff..ea8fa94ebe1b 100644
> --- a/mm/internal.h
> +++ b/mm/internal.h
> @@ -970,8 +970,7 @@ void mlock_folio(struct folio *folio);
> static inline void mlock_vma_folio(struct folio *folio,
> struct vm_area_struct *vma)
> {
> - /* The VM_IO check prevents migration from double-counting during mlock. */
> - if (unlikely((vma->vm_flags & (VM_LOCKED|VM_SPECIAL)) == VM_LOCKED))
> + if (vma_test(vma, VMA_LOCKED_BIT))
> mlock_folio(folio);
> }
>
> @@ -988,7 +987,12 @@ static inline void munlock_vma_folio(struct folio *folio,
> * always munlock the folio and page reclaim will correct it
> * if it's wrong.
> */
> - if (unlikely(vma->vm_flags & VM_LOCKED))
> + /*
> + * VMA_LOCKONFAULT_BIT alone marks an mlock walk in progress, see
> + * mlock_vma_pages_range(). An unmap racing with the walk must still
> + * munlock folios the walk has already counted.
> + */
> + if (unlikely(vma_test_any_mask(vma, VMA_LOCKED_MASK)))
> munlock_folio(folio);
> }
>
> diff --git a/mm/mlock.c b/mm/mlock.c
> index 39215a3eab1f..4235a1518fc9 100644
> --- a/mm/mlock.c
> +++ b/mm/mlock.c
> @@ -316,22 +316,10 @@ static inline unsigned int folio_mlock_step(struct folio *folio,
> return folio_pte_batch(folio, pte, ptent, count);
> }
>
> -static inline bool allow_mlock_munlock(struct folio *folio,
> +static inline bool allow_mlock(struct folio *folio,
> struct vm_area_struct *vma, unsigned long start,
> unsigned long end, unsigned int step)
> {
> - /*
> - * For unlock, allow munlock large folio which is partially
> - * mapped to VMA. As it's possible that large folio is
> - * mlocked and VMA is split later.
> - *
> - * During memory pressure, such kind of large folio can
> - * be split. And the pages are not in VM_LOCKed VMA
> - * can be reclaimed.
> - */
> - if (!vma_test(vma, VMA_LOCKED_BIT))
> - return true;
> -
> /* folio_within_range() cannot take KSM, but any small folio is OK */
> if (!folio_test_large(folio))
> return true;
> @@ -352,6 +340,7 @@ static int mlock_pte_range(pmd_t *pmd, unsigned long addr,
>
> {
> struct vm_area_struct *vma = walk->vma;
> + const bool lock = walk->private;
> spinlock_t *ptl;
> pte_t *start_pte, *pte;
> pte_t ptent;
> @@ -368,7 +357,7 @@ static int mlock_pte_range(pmd_t *pmd, unsigned long addr,
> folio = pmd_folio(*pmd);
> if (folio_is_zone_device(folio))
> goto out;
> - if (vma_test(vma, VMA_LOCKED_BIT))
> + if (lock)
> mlock_folio(folio);
> else
> munlock_folio(folio);
> @@ -390,10 +379,10 @@ static int mlock_pte_range(pmd_t *pmd, unsigned long addr,
> continue;
>
> step = folio_mlock_step(folio, pte, addr, end);
> - if (!allow_mlock_munlock(folio, vma, start, end, step))
> + if (lock && !allow_mlock(folio, vma, start, end, step))
> goto next_entry;
>
> - if (vma_test(vma, VMA_LOCKED_BIT))
> + if (lock)
> mlock_folio(folio);
> else
> munlock_folio(folio);
> @@ -428,31 +417,29 @@ static void mlock_vma_pages_range(struct vm_area_struct *vma,
> .pmd_entry = mlock_pte_range,
> .walk_lock = PGWALK_WRLOCK_VERIFY,
> };
> + const bool lock = vma_flags_test(new_vma_flags, VMA_LOCKED_BIT);
> + vma_flags_t walk_flags = *new_vma_flags;
>
> /*
> - * There is a slight chance that concurrent page migration,
> - * or page reclaim finding a page of this now-VMA_LOCKED_BIT vma,
> - * will call mlock_vma_folio() and raise page's mlock_count:
> - * double counting, leaving the page unevictable indefinitely.
> - * Communicate this danger to mlock_vma_folio() with VMA_IO_BIT,
> - * which is a VMA_SPECIAL_FLAGS flag not allowed on VMA_LOCKED_BIT vmas.
> - * mmap_lock is held in write mode here, so this weird
> - * combination should not be visible to other mmap_lock users;
> - * but WRITE_ONCE so rmap walkers must see VMA_IO_BIT if VMA_LOCKED_BIT.
> + * LOCKONFAULT without LOCKED never otherwise occurs: it marks a walk in
> + * progress so that rmap-side callers, which test VMA_LOCKED_BIT, do not
> + * count folios, while try_to_unmap_one(), which tests VMA_LOCKED_MASK,
> + * still refuses to unmap them.
> */
> - if (vma_flags_test(new_vma_flags, VMA_LOCKED_BIT))
> - vma_flags_set(new_vma_flags, VMA_IO_BIT);
> + if (lock) {
> + vma_flags_clear(&walk_flags, VMA_LOCKED_BIT);
> + vma_flags_set(&walk_flags, VMA_LOCKONFAULT_BIT);
> + }
> +
> vma_start_write(vma);
> - vma_flags_reset_once(vma, new_vma_flags);
> + vma_flags_reset_once(vma, &walk_flags);
>
> lru_add_drain();
> - walk_page_range_vma(vma, start, end, &mlock_walk_ops, NULL);
> + walk_page_range_vma(vma, start, end, &mlock_walk_ops, (void *)lock);
> lru_add_drain();
>
> - if (vma_flags_test(new_vma_flags, VMA_IO_BIT)) {
> - vma_flags_clear(new_vma_flags, VMA_IO_BIT);
> + if (lock)
> vma_flags_reset_once(vma, new_vma_flags);
> - }
> }
>
> /*
> diff --git a/mm/rmap.c b/mm/rmap.c
> index 5332c52909be..b4a4b3929044 100644
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -2239,9 +2239,11 @@ static bool try_to_unmap_one(struct folio *folio, struct vm_area_struct *vma,
>
> /*
> * If the folio is in an mlock()d vma, we must not swap it out.
> + * VMA_LOCKONFAULT_BIT alone marks an mlock walk in progress, see
> + * mlock_vma_pages_range().
> */
> if (!(flags & TTU_IGNORE_MLOCK) &&
> - (vma->vm_flags & VM_LOCKED)) {
> + vma_test_any_mask(vma, VMA_LOCKED_MASK)) {
> ptes++;
>
> /*
> @@ -2395,7 +2397,7 @@ static bool try_to_unmap_one(struct folio *folio, struct vm_area_struct *vma,
> }
> finish_unmap:
> folio_remove_rmap_ptes(folio, page, nr_pages, vma);
> - if (vma->vm_flags & VM_LOCKED)
> + if (vma_test_any_mask(vma, VMA_LOCKED_MASK))
> mlock_drain_local();
> folio_put_refs(folio, nr_pages);
>
> @@ -2770,7 +2772,7 @@ static bool try_to_migrate_one(struct folio *folio, struct vm_area_struct *vma,
> hugetlb_remove_rmap(folio);
> else
> folio_remove_rmap_pte(folio, subpage, vma);
> - if (vma->vm_flags & VM_LOCKED)
> + if (vma_test_any_mask(vma, VMA_LOCKED_MASK))
> mlock_drain_local();
> folio_put(folio);
> }
>
The mlock.c changes are a bit over my head (mostly the old VM_IO handling ...),
but it looks good from what I can tell (hoping Hugh could review ...)
Acked-by: David Hildenbrand (Arm) <david at kernel.org>
--
Cheers,
David
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