Message-ID: <20241120000826.335387-5-surenb@google.com>
Date: Tue, 19 Nov 2024 16:08:25 -0800
From: Suren Baghdasaryan <surenb@...gle.com>
To: akpm@...ux-foundation.org
Cc: willy@...radead.org, liam.howlett@...cle.com, lorenzo.stoakes@...cle.com, 
	mhocko@...e.com, vbabka@...e.cz, hannes@...xchg.org, mjguzik@...il.com, 
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	peterx@...hat.com, oleg@...hat.com, dave@...olabs.net, paulmck@...nel.org, 
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	souravpanda@...gle.com, pasha.tatashin@...een.com, corbet@....net, 
	linux-doc@...r.kernel.org, linux-mm@...ck.org, linux-kernel@...r.kernel.org, 
	kernel-team@...roid.com, surenb@...gle.com
Subject: [PATCH v4 4/5] mm: make vma cache SLAB_TYPESAFE_BY_RCU

To enable SLAB_TYPESAFE_BY_RCU for vma cache we need to ensure that
object reuse before RCU grace period is over will be detected inside
lock_vma_under_rcu().
lock_vma_under_rcu() enters RCU read section, finds the vma at the
given address, locks the vma and checks if it got detached or remapped
to cover a different address range. These last checks are there
to ensure that the vma was not modified after we found it but before
locking it.
vma reuse introduces several new possibilities:
1. vma can be reused after it was found but before it is locked;
2. vma can be reused and reinitialized (including changing its vm_mm)
while being locked in vma_start_read();
3. vma can be reused and reinitialized after it was found but before
it is locked, then attached at a new address or to a new mm while being
read-locked;
For case #1 current checks will help detecting cases when:
- vma was reused but not yet added into the tree (detached check)
- vma was reused at a different address range (address check);
We are missing the check for vm_mm to ensure the reused vma was not
attached to a different mm. This patch adds the missing check.
For case #2, we pass mm to vma_start_read() to prevent access to
unstable vma->vm_mm.
For case #3, we ensure the order in which vma->detached flag and
vm_start/vm_end/vm_mm are set and checked. vma gets attached after
vm_start/vm_end/vm_mm were set and lock_vma_under_rcu() should check
vma->detached before checking vm_start/vm_end/vm_mm. This is required
because attaching vma happens without vma write-lock, as opposed to
vma detaching, which requires vma write-lock. This patch adds memory
barriers inside is_vma_detached() and vma_mark_attached() needed to
order reads and writes to vma->detached vs vm_start/vm_end/vm_mm.
After these provisions, SLAB_TYPESAFE_BY_RCU is added to vm_area_cachep.
This will facilitate vm_area_struct reuse and will minimize the number
of call_rcu() calls.
Adding a freeptr_t into vm_area_struct (unioned with vm_start/vm_end)
could be used to avoids bloating the structure, however currently
custom free pointers are not supported in combination with a ctor
(see the comment for kmem_cache_args.freeptr_offset).

Signed-off-by: Suren Baghdasaryan <surenb@...gle.com>
---
 include/linux/mm.h               | 60 +++++++++++++++++++++++++++-----
 include/linux/mm_types.h         | 13 +++----
 kernel/fork.c                    | 53 +++++++++++++++++-----------
 mm/memory.c                      | 15 +++++---
 mm/vma.c                         |  2 +-
 tools/testing/vma/vma_internal.h |  7 ++--
 6 files changed, 103 insertions(+), 47 deletions(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index dd1b6190df28..2a4794b7a513 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -257,7 +257,7 @@ struct vm_area_struct *vm_area_alloc(struct mm_struct *);
 struct vm_area_struct *vm_area_dup(struct vm_area_struct *);
 void vm_area_free(struct vm_area_struct *);
 /* Use only if VMA has no other users */
-void __vm_area_free(struct vm_area_struct *vma);
+void vm_area_free_unreachable(struct vm_area_struct *vma);
 
 #ifndef CONFIG_MMU
 extern struct rb_root nommu_region_tree;
@@ -690,12 +690,32 @@ static inline void vma_lock_init(struct vm_area_struct *vma)
 	vma->vm_lock_seq = UINT_MAX;
 }
 
+#define VMA_BEFORE_LOCK		offsetof(struct vm_area_struct, vm_lock)
+#define VMA_LOCK_END(vma)	\
+	(((void *)(vma)) + offsetofend(struct vm_area_struct, vm_lock))
+#define VMA_AFTER_LOCK		\
+	(sizeof(struct vm_area_struct) - offsetofend(struct vm_area_struct, vm_lock))
+
+static inline void vma_clear(struct vm_area_struct *vma)
+{
+	/* Preserve vma->vm_lock */
+	memset(vma, 0, VMA_BEFORE_LOCK);
+	memset(VMA_LOCK_END(vma), 0, VMA_AFTER_LOCK);
+}
+
+static inline void vma_copy(struct vm_area_struct *new, struct vm_area_struct *orig)
+{
+	/* Preserve vma->vm_lock */
+	data_race(memcpy(new, orig, VMA_BEFORE_LOCK));
+	data_race(memcpy(VMA_LOCK_END(new), VMA_LOCK_END(orig), VMA_AFTER_LOCK));
+}
+
 /*
  * Try to read-lock a vma. The function is allowed to occasionally yield false
  * locked result to avoid performance overhead, in which case we fall back to
  * using mmap_lock. The function should never yield false unlocked result.
  */
-static inline bool vma_start_read(struct vm_area_struct *vma)
+static inline bool vma_start_read(struct mm_struct *mm, struct vm_area_struct *vma)
 {
 	/*
 	 * Check before locking. A race might cause false locked result.
@@ -704,7 +724,7 @@ static inline bool vma_start_read(struct vm_area_struct *vma)
 	 * we don't rely on for anything - the mm_lock_seq read against which we
 	 * need ordering is below.
 	 */
-	if (READ_ONCE(vma->vm_lock_seq) == READ_ONCE(vma->vm_mm->mm_lock_seq.sequence))
+	if (READ_ONCE(vma->vm_lock_seq) == READ_ONCE(mm->mm_lock_seq.sequence))
 		return false;
 
 	if (unlikely(down_read_trylock(&vma->vm_lock.lock) == 0))
@@ -721,7 +741,7 @@ static inline bool vma_start_read(struct vm_area_struct *vma)
 	 * after it has been unlocked.
 	 * This pairs with RELEASE semantics in vma_end_write_all().
 	 */
-	if (unlikely(vma->vm_lock_seq == raw_read_seqcount(&vma->vm_mm->mm_lock_seq))) {
+	if (unlikely(vma->vm_lock_seq == raw_read_seqcount(&mm->mm_lock_seq))) {
 		up_read(&vma->vm_lock.lock);
 		return false;
 	}
@@ -810,7 +830,15 @@ static inline void vma_assert_locked(struct vm_area_struct *vma)
 
 static inline void vma_mark_attached(struct vm_area_struct *vma)
 {
-	vma->detached = false;
+	/*
+	 * This pairs with smp_rmb() inside is_vma_detached().
+	 * vma is marked attached after all vma modifications are done and it
+	 * got added into the vma tree. All prior vma modifications should be
+	 * made visible before marking the vma attached.
+	 */
+	smp_wmb();
+	/* This pairs with READ_ONCE() in is_vma_detached(). */
+	WRITE_ONCE(vma->detached, false);
 }
 
 static inline void vma_mark_detached(struct vm_area_struct *vma)
@@ -822,7 +850,18 @@ static inline void vma_mark_detached(struct vm_area_struct *vma)
 
 static inline bool is_vma_detached(struct vm_area_struct *vma)
 {
-	return vma->detached;
+	bool detached;
+
+	/* This pairs with WRITE_ONCE() in vma_mark_attached(). */
+	detached = READ_ONCE(vma->detached);
+	/*
+	 * This pairs with smp_wmb() inside vma_mark_attached() to ensure
+	 * vma->detached is read before vma attributes read later inside
+	 * lock_vma_under_rcu().
+	 */
+	smp_rmb();
+
+	return detached;
 }
 
 static inline void release_fault_lock(struct vm_fault *vmf)
@@ -847,7 +886,11 @@ struct vm_area_struct *lock_vma_under_rcu(struct mm_struct *mm,
 #else /* CONFIG_PER_VMA_LOCK */
 
 static inline void vma_lock_init(struct vm_area_struct *vma) {}
-static inline bool vma_start_read(struct vm_area_struct *vma)
+static inline void vma_clear(struct vm_area_struct *vma)
+		{ memset(vma, 0, sizeof(*vma)); }
+static inline void vma_copy(struct vm_area_struct *new, struct vm_area_struct *orig)
+		{ data_race(memcpy(new, orig, sizeof(*new))); }
+static inline bool vma_start_read(struct mm_struct *mm, struct vm_area_struct *vma)
 		{ return false; }
 static inline void vma_end_read(struct vm_area_struct *vma) {}
 static inline void vma_start_write(struct vm_area_struct *vma) {}
@@ -883,7 +926,7 @@ extern const struct vm_operations_struct vma_dummy_vm_ops;
 
 static inline void vma_init(struct vm_area_struct *vma, struct mm_struct *mm)
 {
-	memset(vma, 0, sizeof(*vma));
+	vma_clear(vma);
 	vma->vm_mm = mm;
 	vma->vm_ops = &vma_dummy_vm_ops;
 	INIT_LIST_HEAD(&vma->anon_vma_chain);
@@ -892,7 +935,6 @@ static inline void vma_init(struct vm_area_struct *vma, struct mm_struct *mm)
 	vma->detached = true;
 #endif
 	vma_numab_state_init(vma);
-	vma_lock_init(vma);
 }
 
 /* Use when VMA is not part of the VMA tree and needs no locking */
diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index 5c4bfdcfac72..8f6b0c935c2b 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -667,15 +667,10 @@ struct vma_numab_state {
 struct vm_area_struct {
 	/* The first cache line has the info for VMA tree walking. */
 
-	union {
-		struct {
-			/* VMA covers [vm_start; vm_end) addresses within mm */
-			unsigned long vm_start;
-			unsigned long vm_end;
-		};
-#ifdef CONFIG_PER_VMA_LOCK
-		struct rcu_head vm_rcu;	/* Used for deferred freeing. */
-#endif
+	struct {
+		/* VMA covers [vm_start; vm_end) addresses within mm */
+		unsigned long vm_start;
+		unsigned long vm_end;
 	};
 
 	/*
diff --git a/kernel/fork.c b/kernel/fork.c
index f0cec673583c..76c68b041f8a 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -436,6 +436,11 @@ static struct kmem_cache *vm_area_cachep;
 /* SLAB cache for mm_struct structures (tsk->mm) */
 static struct kmem_cache *mm_cachep;
 
+static void vm_area_ctor(void *data)
+{
+	vma_lock_init(data);
+}
+
 struct vm_area_struct *vm_area_alloc(struct mm_struct *mm)
 {
 	struct vm_area_struct *vma;
@@ -462,8 +467,7 @@ struct vm_area_struct *vm_area_dup(struct vm_area_struct *orig)
 	 * orig->shared.rb may be modified concurrently, but the clone
 	 * will be reinitialized.
 	 */
-	data_race(memcpy(new, orig, sizeof(*new)));
-	vma_lock_init(new);
+	vma_copy(new, orig);
 	INIT_LIST_HEAD(&new->anon_vma_chain);
 #ifdef CONFIG_PER_VMA_LOCK
 	/* vma is not locked, can't use vma_mark_detached() */
@@ -475,32 +479,37 @@ struct vm_area_struct *vm_area_dup(struct vm_area_struct *orig)
 	return new;
 }
 
-void __vm_area_free(struct vm_area_struct *vma)
+static void __vm_area_free(struct vm_area_struct *vma, bool unreachable)
 {
+#ifdef CONFIG_PER_VMA_LOCK
+	/*
+	 * With SLAB_TYPESAFE_BY_RCU, vma can be reused and we need
+	 * vma->detached to be set before vma is returned into the cache.
+	 * This way reused object won't be used by readers until it's
+	 * initialized and reattached.
+	 * If vma is unreachable, there can be no other users and we
+	 * can set vma->detached directly with no risk of a race.
+	 * If vma is reachable, then it should have been already detached
+	 * under vma write-lock or it was never attached.
+	 */
+	if (unreachable)
+		vma->detached = true;
+	else
+		VM_BUG_ON_VMA(!is_vma_detached(vma), vma);
+#endif
 	vma_numab_state_free(vma);
 	free_anon_vma_name(vma);
 	kmem_cache_free(vm_area_cachep, vma);
 }
 
-#ifdef CONFIG_PER_VMA_LOCK
-static void vm_area_free_rcu_cb(struct rcu_head *head)
+void vm_area_free(struct vm_area_struct *vma)
 {
-	struct vm_area_struct *vma = container_of(head, struct vm_area_struct,
-						  vm_rcu);
-
-	/* The vma should not be locked while being destroyed. */
-	VM_BUG_ON_VMA(rwsem_is_locked(&vma->vm_lock.lock), vma);
-	__vm_area_free(vma);
+	__vm_area_free(vma, false);
 }
-#endif
 
-void vm_area_free(struct vm_area_struct *vma)
+void vm_area_free_unreachable(struct vm_area_struct *vma)
 {
-#ifdef CONFIG_PER_VMA_LOCK
-	call_rcu(&vma->vm_rcu, vm_area_free_rcu_cb);
-#else
-	__vm_area_free(vma);
-#endif
+	__vm_area_free(vma, true);
 }
 
 static void account_kernel_stack(struct task_struct *tsk, int account)
@@ -3135,9 +3144,11 @@ void __init proc_caches_init(void)
 			sizeof(struct fs_struct), 0,
 			SLAB_HWCACHE_ALIGN|SLAB_PANIC|SLAB_ACCOUNT,
 			NULL);
-	vm_area_cachep = KMEM_CACHE(vm_area_struct,
-			SLAB_HWCACHE_ALIGN|SLAB_NO_MERGE|SLAB_PANIC|
-			SLAB_ACCOUNT);
+	vm_area_cachep = kmem_cache_create("vm_area_struct",
+			sizeof(struct vm_area_struct), 0,
+			SLAB_HWCACHE_ALIGN|SLAB_PANIC|SLAB_TYPESAFE_BY_RCU|
+			SLAB_ACCOUNT, vm_area_ctor);
+
 	mmap_init();
 	nsproxy_cache_init();
 }
diff --git a/mm/memory.c b/mm/memory.c
index d0197a0c0996..b5fbc71b46bd 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -6275,10 +6275,16 @@ struct vm_area_struct *lock_vma_under_rcu(struct mm_struct *mm,
 	if (!vma)
 		goto inval;
 
-	if (!vma_start_read(vma))
+	if (!vma_start_read(mm, vma))
 		goto inval;
 
-	/* Check if the VMA got isolated after we found it */
+	/*
+	 * Check if the VMA got isolated after we found it.
+	 * Note: vma we found could have been recycled and is being reattached.
+	 * It's possible to attach a vma while it is read-locked, however a
+	 * read-locked vma can't be detached (detaching requires write-locking).
+	 * Therefore if this check passes, we have an attached and stable vma.
+	 */
 	if (is_vma_detached(vma)) {
 		vma_end_read(vma);
 		count_vm_vma_lock_event(VMA_LOCK_MISS);
@@ -6292,8 +6298,9 @@ struct vm_area_struct *lock_vma_under_rcu(struct mm_struct *mm,
 	 * fields are accessible for RCU readers.
 	 */
 
-	/* Check since vm_start/vm_end might change before we lock the VMA */
-	if (unlikely(address < vma->vm_start || address >= vma->vm_end))
+	/* Check if the vma we locked is the right one. */
+	if (unlikely(vma->vm_mm != mm ||
+		     address < vma->vm_start || address >= vma->vm_end))
 		goto inval_end_read;
 
 	rcu_read_unlock();
diff --git a/mm/vma.c b/mm/vma.c
index 73104d434567..050b83df3df2 100644
--- a/mm/vma.c
+++ b/mm/vma.c
@@ -382,7 +382,7 @@ void remove_vma(struct vm_area_struct *vma, bool unreachable)
 		fput(vma->vm_file);
 	mpol_put(vma_policy(vma));
 	if (unreachable)
-		__vm_area_free(vma);
+		vm_area_free_unreachable(vma);
 	else
 		vm_area_free(vma);
 }
diff --git a/tools/testing/vma/vma_internal.h b/tools/testing/vma/vma_internal.h
index 2fed366d20ef..fd668d6cafc0 100644
--- a/tools/testing/vma/vma_internal.h
+++ b/tools/testing/vma/vma_internal.h
@@ -632,14 +632,15 @@ static inline void mpol_put(struct mempolicy *)
 {
 }
 
-static inline void __vm_area_free(struct vm_area_struct *vma)
+static inline void vm_area_free(struct vm_area_struct *vma)
 {
 	free(vma);
 }
 
-static inline void vm_area_free(struct vm_area_struct *vma)
+static inline void vm_area_free_unreachable(struct vm_area_struct *vma)
 {
-	__vm_area_free(vma);
+	vma->detached = true;
+	vm_area_free(vma);
 }
 
 static inline void lru_add_drain(void)
-- 
2.47.0.338.g60cca15819-goog

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