Re: [PATCH v5 23/38] mm/mlock: eliminate weird VMA_IO_BIT abuse and simplify

["David Hildenbrand (Arm)" <david@kernel.org>]

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@nvidia.com>
> Signed-off-by: Lorenzo Stoakes (ARM) <ljs@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@kernel.org>

-- 
Cheers,

David