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
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. This might lead to vma_start_read() returning
a false locked result but that's not critical if it's rare because
it will only lead to a retry under mmap_lock.
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.
Signed-off-by: Suren Baghdasaryan <surenb@xxxxxxxxxx>
---
include/linux/mm.h | 36 +++++--
include/linux/mm_types.h | 10 +-
include/linux/slab.h | 6 --
kernel/fork.c | 157 +++++++++++++++++++++++++------
mm/memory.c | 15 ++-
mm/vma.c | 2 +-
tools/testing/vma/vma_internal.h | 7 +-
7 files changed, 179 insertions(+), 54 deletions(-)
diff --git a/include/linux/mm.h b/include/linux/mm.h
index 2bf38c1e9cca..3568bcbc7c81 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;
@@ -706,8 +706,10 @@ static inline void vma_lock_init(struct vm_area_struct *vma)
* 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.
+ * False locked result is possible if mm_lock_seq overflows or if vma gets
+ * reused and attached to a different mm before we lock it.
*/
-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.
@@ -716,7 +718,7 @@ 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))
@@ -733,7 +735,7 @@ 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;
}
@@ -822,7 +824,15 @@ 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)
@@ -834,7 +844,18 @@ 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)
@@ -859,7 +880,7 @@ 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 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) {}
@@ -893,6 +914,7 @@ static inline void assert_fault_locked(struct vm_fault *vmf)
extern const struct vm_operations_struct vma_dummy_vm_ops;
+/* Use on VMAs not created using vm_area_alloc() */
static inline void vma_init(struct vm_area_struct *vma, struct mm_struct *mm)
{
memset(vma, 0, sizeof(*vma));
diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index be3551654325..5d8779997266 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -543,6 +543,12 @@ static inline void *folio_get_private(struct folio *folio)
typedef unsigned long vm_flags_t;
+/*
+ * freeptr_t represents a SLUB freelist pointer, which might be encoded
+ * and not dereferenceable if CONFIG_SLAB_FREELIST_HARDENED is enabled.
+ */
+typedef struct { unsigned long v; } freeptr_t;
+
/*
* A region containing a mapping of a non-memory backed file under NOMMU
* conditions. These are held in a global tree and are pinned by the VMAs that
@@ -657,9 +663,7 @@ struct vm_area_struct {
unsigned long vm_start;
unsigned long vm_end;
};
-#ifdef CONFIG_PER_VMA_LOCK
- struct rcu_head vm_rcu; /* Used for deferred freeing. */
-#endif
+ freeptr_t vm_freeptr; /* Pointer used by SLAB_TYPESAFE_BY_RCU */
};
/*
diff --git a/include/linux/slab.h b/include/linux/slab.h
index 10a971c2bde3..681b685b6c4e 100644
--- a/include/linux/slab.h
+++ b/include/linux/slab.h
@@ -234,12 +234,6 @@ enum _slab_flag_bits {
#define SLAB_NO_OBJ_EXT __SLAB_FLAG_UNUSED
#endif
-/*
- * freeptr_t represents a SLUB freelist pointer, which might be encoded
- * and not dereferenceable if CONFIG_SLAB_FREELIST_HARDENED is enabled.
- */
-typedef struct { unsigned long v; } freeptr_t;
-
/*
* ZERO_SIZE_PTR will be returned for zero sized kmalloc requests.
*
diff --git a/kernel/fork.c b/kernel/fork.c
index 71990f46aa4e..e7e76a660e4c 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -436,6 +436,98 @@ 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)
+{
+ struct vm_area_struct *vma = (struct vm_area_struct *)data;
+
+#ifdef CONFIG_PER_VMA_LOCK
+ /* vma is not locked, can't use vma_mark_detached() */
+ vma->detached = true;
+#endif
+ INIT_LIST_HEAD(&vma->anon_vma_chain);
+ vma_lock_init(vma);
+}
+
+#ifdef CONFIG_PER_VMA_LOCK
+
+static void vma_clear(struct vm_area_struct *vma, struct mm_struct *mm)
+{
+ vma->vm_mm = mm;
+ vma->vm_ops = &vma_dummy_vm_ops;
+ vma->vm_start = 0;
+ vma->vm_end = 0;
+ vma->anon_vma = NULL;
+ vma->vm_pgoff = 0;
+ vma->vm_file = NULL;
+ vma->vm_private_data = NULL;
+ vm_flags_init(vma, 0);
+ memset(&vma->vm_page_prot, 0, sizeof(vma->vm_page_prot));
+ memset(&vma->shared, 0, sizeof(vma->shared));
+ memset(&vma->vm_userfaultfd_ctx, 0, sizeof(vma->vm_userfaultfd_ctx));
+ vma_numab_state_init(vma);
+#ifdef CONFIG_ANON_VMA_NAME
+ vma->anon_name = NULL;
+#endif
+#ifdef CONFIG_SWAP
+ memset(&vma->swap_readahead_info, 0, sizeof(vma->swap_readahead_info));
+#endif
+#ifndef CONFIG_MMU
+ vma->vm_region = NULL;
+#endif
+#ifdef CONFIG_NUMA
+ vma->vm_policy = NULL;
+#endif
+}
+
+static void vma_copy(const struct vm_area_struct *src, struct vm_area_struct *dest)
+{
+ dest->vm_mm = src->vm_mm;
+ dest->vm_ops = src->vm_ops;
+ dest->vm_start = src->vm_start;
+ dest->vm_end = src->vm_end;
+ dest->anon_vma = src->anon_vma;
+ dest->vm_pgoff = src->vm_pgoff;
+ dest->vm_file = src->vm_file;
+ dest->vm_private_data = src->vm_private_data;
+ vm_flags_init(dest, src->vm_flags);
+ memcpy(&dest->vm_page_prot, &src->vm_page_prot,
+ sizeof(dest->vm_page_prot));
+ memcpy(&dest->shared, &src->shared, sizeof(dest->shared));
+ memcpy(&dest->vm_userfaultfd_ctx, &src->vm_userfaultfd_ctx,
+ sizeof(dest->vm_userfaultfd_ctx));
+#ifdef CONFIG_ANON_VMA_NAME
+ dest->anon_name = src->anon_name;
+#endif
+#ifdef CONFIG_SWAP
+ memcpy(&dest->swap_readahead_info, &src->swap_readahead_info,
+ sizeof(dest->swap_readahead_info));
+#endif
+#ifndef CONFIG_MMU
+ dest->vm_region = src->vm_region;
+#endif
+#ifdef CONFIG_NUMA
+ dest->vm_policy = src->vm_policy;
+#endif
+}
+
+#else /* CONFIG_PER_VMA_LOCK */
+
+static void vma_clear(struct vm_area_struct *vma, struct mm_struct *mm)
+{
+ vma_init(vma, mm);
+}
+
+static void vma_copy(const struct vm_area_struct *src, struct vm_area_struct *dest)
+{
+ /*
+ * orig->shared.rb may be modified concurrently, but the clone
+ * will be reinitialized.
+ */
+ data_race(memcpy(dest, src, sizeof(*dest)));
+}
+
+#endif /* CONFIG_PER_VMA_LOCK */
+
struct vm_area_struct *vm_area_alloc(struct mm_struct *mm)
{
struct vm_area_struct *vma;
@@ -444,7 +536,7 @@ struct vm_area_struct *vm_area_alloc(struct mm_struct *mm)
if (!vma)
return NULL;
- vma_init(vma, mm);
+ vma_clear(vma, mm);
return vma;
}
@@ -458,49 +550,46 @@ struct vm_area_struct *vm_area_dup(struct vm_area_struct *orig)
ASSERT_EXCLUSIVE_WRITER(orig->vm_flags);
ASSERT_EXCLUSIVE_WRITER(orig->vm_file);
- /*
- * orig->shared.rb may be modified concurrently, but the clone
- * will be reinitialized.
- */
- data_race(memcpy(new, orig, sizeof(*new)));
- vma_lock_init(new);
- INIT_LIST_HEAD(&new->anon_vma_chain);
-#ifdef CONFIG_PER_VMA_LOCK
- /* vma is not locked, can't use vma_mark_detached() */
- new->detached = true;
-#endif
+ vma_copy(orig, new);
vma_numab_state_init(new);
dup_anon_vma_name(orig, new);
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);
+ vma->vm_lock_seq = UINT_MAX;
+#endif
+ VM_BUG_ON_VMA(!list_empty(&vma->anon_vma_chain), vma);
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)
@@ -3141,6 +3230,12 @@ void __init mm_cache_init(void)
void __init proc_caches_init(void)
{
+ struct kmem_cache_args args = {
+ .use_freeptr_offset = true,
+ .freeptr_offset = offsetof(struct vm_area_struct, vm_freeptr),
+ .ctor = vm_area_ctor,
+ };
+
sighand_cachep = kmem_cache_create("sighand_cache",
sizeof(struct sighand_struct), 0,
SLAB_HWCACHE_ALIGN|SLAB_PANIC|SLAB_TYPESAFE_BY_RCU|
@@ -3157,9 +3252,11 @@ 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|
+ vm_area_cachep = kmem_cache_create("vm_area_struct",
+ sizeof(struct vm_area_struct), &args,
+ SLAB_HWCACHE_ALIGN|SLAB_PANIC|SLAB_TYPESAFE_BY_RCU|
SLAB_ACCOUNT);
+
mmap_init();
nsproxy_cache_init();
}
diff --git a/mm/memory.c b/mm/memory.c
index b252f19b28c9..6f4d4d423835 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -6368,10 +6368,16 @@ 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);
@@ -6385,8 +6391,9 @@ 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 cdc63728f47f..648784416833 100644
--- a/mm/vma.c
+++ b/mm/vma.c
@@ -414,7 +414,7 @@ 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 0cdc5f8c3d60..3eeb1317cc69 100644
--- a/tools/testing/vma/vma_internal.h
+++ b/tools/testing/vma/vma_internal.h
@@ -685,14 +685,15 @@ 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
