[PATCH 23/39] mm/mlock: eliminate weird VMA_IO_BIT abuse and simplify

["Lorenzo Stoakes (ARM)" <ljs@kernel.org>]

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.

Finally, with the other mlock abuse of VMA_IO_BIT addressed, update
mlock_vma_folio(), munlock_vma_folio() and folio_add_lru_vma() to simply
test for VMA_LOCKED_BIT.

While here, also replace some deprecated VMA flag predicates.

Signed-off-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>
---
 mm/folio.c    |  2 +-
 mm/internal.h |  5 ++---
 mm/mlock.c    | 51 +++++++++++++++++++--------------------------------
 mm/rmap.c     |  4 +++-
 4 files changed, 25 insertions(+), 37 deletions(-)

diff --git a/mm/folio.c b/mm/folio.c
index 50a6dbe55998..a3f5c463f665 100644
--- a/mm/folio.c
+++ b/mm/folio.c
@@ -502,7 +502,7 @@ 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 6e27d3b10c01..04b1f1d3d960 100644
--- a/mm/internal.h
+++ b/mm/internal.h
@@ -975,8 +975,7 @@ 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);
 }
 
@@ -993,7 +992,7 @@ 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))
+	if (unlikely(vma_test(vma, VMA_LOCKED_BIT)))
 		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 5fefe5b060b1..120c894d2dde 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++;
 
 			/*

-- 
2.55.0