The patch titled
Subject: mm: replace vm_lock and detached flag with a reference count
has been added to the -mm mm-unstable branch. Its filename is
mm-replace-vm_lock-and-detached-flag-with-a-reference-count.patch
This patch will shortly appear at
https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mm-replace-vm_lock-and-detached-flag-with-a-reference-count.patch
This patch will later appear in the mm-unstable branch at
git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm
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*** Remember to use Documentation/process/submit-checklist.rst when testing your code ***
The -mm tree is included into linux-next via the mm-everything
branch at git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm
and is updated there every 2-3 working days
------------------------------------------------------
From: Suren Baghdasaryan <surenb@xxxxxxxxxx>
Subject: mm: replace vm_lock and detached flag with a reference count
Date: Fri, 10 Jan 2025 20:25:58 -0800
rw_semaphore is a sizable structure of 40 bytes and consumes considerable
space for each vm_area_struct. However vma_lock has two important
specifics which can be used to replace rw_semaphore with a simpler
structure:
1. Readers never wait. They try to take the vma_lock and fall back to
mmap_lock if that fails.
2. Only one writer at a time will ever try to write-lock a vma_lock
because writers first take mmap_lock in write mode. Because of these
requirements, full rw_semaphore functionality is not needed and we can
replace rw_semaphore and the vma->detached flag with a refcount
(vm_refcnt).
When vma is in detached state, vm_refcnt is 0 and only a call to
vma_mark_attached() can take it out of this state. Note that unlike
before, now we enforce both vma_mark_attached() and vma_mark_detached() to
be done only after vma has been write-locked. vma_mark_attached() changes
vm_refcnt to 1 to indicate that it has been attached to the vma tree.
When a reader takes read lock, it increments vm_refcnt, unless the top
usable bit of vm_refcnt (0x40000000) is set, indicating presence of a
writer. When writer takes write lock, it sets the top usable bit to
indicate its presence. If there are readers, writer will wait using newly
introduced mm->vma_writer_wait. Since all writers take mmap_lock in write
mode first, there can be only one writer at a time. The last reader to
release the lock will signal the writer to wake up. refcount might
overflow if there are many competing readers, in which case read-locking
will fail. Readers are expected to handle such failures. In summary:
1. all readers increment the vm_refcnt;
2. writer sets top usable (writer) bit of vm_refcnt;
3. readers cannot increment the vm_refcnt if the writer bit is set;
4. in the presence of readers, writer must wait for the vm_refcnt to drop
to 1 (ignoring the writer bit), indicating an attached vma with no readers;
5. vm_refcnt overflow is handled by the readers.
While this vm_lock replacement does not yet result in a smaller
vm_area_struct (it stays at 256 bytes due to cacheline alignment), it
allows for further size optimization by structure member regrouping
to bring the size of vm_area_struct below 192 bytes.
Link: https://lkml.kernel.org/r/20250111042604.3230628-12-surenb@xxxxxxxxxx
Signed-off-by: Suren Baghdasaryan <surenb@xxxxxxxxxx>
Suggested-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Suggested-by: Matthew Wilcox <willy@xxxxxxxxxxxxx>
Reviewed-by: Vlastimil Babka <vbabka@xxxxxxx>
Cc: Christian Brauner <brauner@xxxxxxxxxx>
Cc: David Hildenbrand <david@xxxxxxxxxx>
Cc: David Howells <dhowells@xxxxxxxxxx>
Cc: Davidlohr Bueso <dave@xxxxxxxxxxxx>
Cc: Hillf Danton <hdanton@xxxxxxxx>
Cc: Hugh Dickens <hughd@xxxxxxxxxx>
Cc: Jann Horn <jannh@xxxxxxxxxx>
Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
Cc: Jonathan Corbet <corbet@xxxxxxx>
Cc: kernel test robot <oliver.sang@xxxxxxxxx>
Cc: Klara Modin <klarasmodin@xxxxxxxxx>
Cc: Liam R. Howlett <Liam.Howlett@xxxxxxxxxx>
Cc: Lokesh Gidra <lokeshgidra@xxxxxxxxxx>
Cc: Lorenzo Stoakes <lorenzo.stoakes@xxxxxxxxxx>
Cc: Mateusz Guzik <mjguzik@xxxxxxxxx>
Cc: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
Cc: Michal Hocko <mhocko@xxxxxxxx>
Cc: Minchan Kim <minchan@xxxxxxxxxx>
Cc: Oleg Nesterov <oleg@xxxxxxxxxx>
Cc: Pasha Tatashin <pasha.tatashin@xxxxxxxxxx>
Cc: "Paul E . McKenney" <paulmck@xxxxxxxxxx>
Cc: Peter Xu <peterx@xxxxxxxxxx>
Cc: Shakeel Butt <shakeel.butt@xxxxxxxxx>
Cc: Sourav Panda <souravpanda@xxxxxxxxxx>
Cc: Wei Yang <richard.weiyang@xxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
include/linux/mm.h | 102 +++++++++++++++++++----------
include/linux/mm_types.h | 22 ++----
kernel/fork.c | 13 +--
mm/init-mm.c | 1
mm/memory.c | 80 +++++++++++++++++++---
tools/testing/vma/linux/atomic.h | 5 +
tools/testing/vma/vma_internal.h | 66 ++++++++++--------
7 files changed, 198 insertions(+), 91 deletions(-)
--- a/include/linux/mm.h~mm-replace-vm_lock-and-detached-flag-with-a-reference-count
+++ a/include/linux/mm.h
@@ -32,6 +32,7 @@
#include <linux/memremap.h>
#include <linux/slab.h>
#include <linux/cacheinfo.h>
+#include <linux/rcuwait.h>
struct mempolicy;
struct anon_vma;
@@ -697,12 +698,43 @@ static inline void vma_numab_state_free(
#endif /* CONFIG_NUMA_BALANCING */
#ifdef CONFIG_PER_VMA_LOCK
-static inline void vma_lock_init(struct vm_area_struct *vma)
+static inline void vma_lock_init(struct vm_area_struct *vma, bool reset_refcnt)
{
- init_rwsem(&vma->vm_lock.lock);
+#ifdef CONFIG_DEBUG_LOCK_ALLOC
+ static struct lock_class_key lockdep_key;
+
+ lockdep_init_map(&vma->vmlock_dep_map, "vm_lock", &lockdep_key, 0);
+#endif
+ if (reset_refcnt)
+ refcount_set(&vma->vm_refcnt, 0);
vma->vm_lock_seq = UINT_MAX;
}
+static inline bool is_vma_writer_only(int refcnt)
+{
+ /*
+ * With a writer and no readers, refcnt is VMA_LOCK_OFFSET if the vma
+ * is detached and (VMA_LOCK_OFFSET + 1) if it is attached. Waiting on
+ * a detached vma happens only in vma_mark_detached() and is a rare
+ * case, therefore most of the time there will be no unnecessary wakeup.
+ */
+ return refcnt & VMA_LOCK_OFFSET && refcnt <= VMA_LOCK_OFFSET + 1;
+}
+
+static inline void vma_refcount_put(struct vm_area_struct *vma)
+{
+ /* Use a copy of vm_mm in case vma is freed after we drop vm_refcnt */
+ struct mm_struct *mm = vma->vm_mm;
+ int oldcnt;
+
+ rwsem_release(&vma->vmlock_dep_map, _RET_IP_);
+ if (!__refcount_dec_and_test(&vma->vm_refcnt, &oldcnt)) {
+
+ if (is_vma_writer_only(oldcnt - 1))
+ rcuwait_wake_up(&mm->vma_writer_wait);
+ }
+}
+
/*
* 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
@@ -710,6 +742,8 @@ static inline void vma_lock_init(struct
*/
static inline bool vma_start_read(struct vm_area_struct *vma)
{
+ int oldcnt;
+
/*
* Check before locking. A race might cause false locked result.
* We can use READ_ONCE() for the mm_lock_seq here, and don't need
@@ -720,13 +754,19 @@ static inline bool vma_start_read(struct
if (READ_ONCE(vma->vm_lock_seq) == READ_ONCE(vma->vm_mm->mm_lock_seq.sequence))
return false;
- if (unlikely(down_read_trylock(&vma->vm_lock.lock) == 0))
+ /*
+ * If VMA_LOCK_OFFSET is set, __refcount_inc_not_zero_limited() will fail
+ * because VMA_REF_LIMIT is less than VMA_LOCK_OFFSET.
+ */
+ if (unlikely(!__refcount_inc_not_zero_limited(&vma->vm_refcnt, &oldcnt,
+ VMA_REF_LIMIT)))
return false;
+ rwsem_acquire_read(&vma->vmlock_dep_map, 0, 1, _RET_IP_);
/*
- * Overflow might produce false locked result.
+ * Overflow of vm_lock_seq/mm_lock_seq might produce false locked result.
* False unlocked result is impossible because we modify and check
- * vma->vm_lock_seq under vma->vm_lock protection and mm->mm_lock_seq
+ * vma->vm_lock_seq under vma->vm_refcnt protection and mm->mm_lock_seq
* modification invalidates all existing locks.
*
* We must use ACQUIRE semantics for the mm_lock_seq so that if we are
@@ -735,9 +775,10 @@ static inline bool vma_start_read(struct
* 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))) {
- up_read(&vma->vm_lock.lock);
+ vma_refcount_put(vma);
return false;
}
+
return true;
}
@@ -749,8 +790,14 @@ static inline bool vma_start_read(struct
*/
static inline bool vma_start_read_locked_nested(struct vm_area_struct *vma, int subclass)
{
+ int oldcnt;
+
mmap_assert_locked(vma->vm_mm);
- down_read_nested(&vma->vm_lock.lock, subclass);
+ if (unlikely(!__refcount_inc_not_zero_limited(&vma->vm_refcnt, &oldcnt,
+ VMA_REF_LIMIT)))
+ return false;
+
+ rwsem_acquire_read(&vma->vmlock_dep_map, 0, 1, _RET_IP_);
return true;
}
@@ -762,16 +809,12 @@ static inline bool vma_start_read_locked
*/
static inline bool vma_start_read_locked(struct vm_area_struct *vma)
{
- mmap_assert_locked(vma->vm_mm);
- down_read(&vma->vm_lock.lock);
- return true;
+ return vma_start_read_locked_nested(vma, 0);
}
static inline void vma_end_read(struct vm_area_struct *vma)
{
- rcu_read_lock(); /* keeps vma alive till the end of up_read */
- up_read(&vma->vm_lock.lock);
- rcu_read_unlock();
+ vma_refcount_put(vma);
}
/* WARNING! Can only be used if mmap_lock is expected to be write-locked */
@@ -813,36 +856,33 @@ static inline void vma_assert_write_lock
static inline void vma_assert_locked(struct vm_area_struct *vma)
{
- if (!rwsem_is_locked(&vma->vm_lock.lock))
+ if (refcount_read(&vma->vm_refcnt) <= 1)
vma_assert_write_locked(vma);
}
+/*
+ * WARNING: to avoid racing with vma_mark_attached()/vma_mark_detached(), these
+ * assertions should be made either under mmap_write_lock or when the object
+ * has been isolated under mmap_write_lock, ensuring no competing writers.
+ */
static inline void vma_assert_attached(struct vm_area_struct *vma)
{
- VM_BUG_ON_VMA(vma->detached, vma);
+ VM_BUG_ON_VMA(!refcount_read(&vma->vm_refcnt), vma);
}
static inline void vma_assert_detached(struct vm_area_struct *vma)
{
- VM_BUG_ON_VMA(!vma->detached, vma);
+ VM_BUG_ON_VMA(refcount_read(&vma->vm_refcnt), vma);
}
static inline void vma_mark_attached(struct vm_area_struct *vma)
{
- vma->detached = false;
-}
-
-static inline void vma_mark_detached(struct vm_area_struct *vma)
-{
- /* When detaching vma should be write-locked */
vma_assert_write_locked(vma);
- vma->detached = true;
+ vma_assert_detached(vma);
+ refcount_set(&vma->vm_refcnt, 1);
}
-static inline bool is_vma_detached(struct vm_area_struct *vma)
-{
- return vma->detached;
-}
+void vma_mark_detached(struct vm_area_struct *vma);
static inline void release_fault_lock(struct vm_fault *vmf)
{
@@ -865,7 +905,7 @@ struct vm_area_struct *lock_vma_under_rc
#else /* CONFIG_PER_VMA_LOCK */
-static inline void vma_lock_init(struct vm_area_struct *vma) {}
+static inline void vma_lock_init(struct vm_area_struct *vma, bool reset_refcnt) {}
static inline bool vma_start_read(struct vm_area_struct *vma)
{ return false; }
static inline void vma_end_read(struct vm_area_struct *vma) {}
@@ -908,12 +948,8 @@ static inline void vma_init(struct vm_ar
vma->vm_mm = mm;
vma->vm_ops = &vma_dummy_vm_ops;
INIT_LIST_HEAD(&vma->anon_vma_chain);
-#ifdef CONFIG_PER_VMA_LOCK
- /* vma is not locked, can't use vma_mark_detached() */
- vma->detached = true;
-#endif
vma_numab_state_init(vma);
- vma_lock_init(vma);
+ vma_lock_init(vma, false);
}
/* Use when VMA is not part of the VMA tree and needs no locking */
--- a/include/linux/mm_types.h~mm-replace-vm_lock-and-detached-flag-with-a-reference-count
+++ a/include/linux/mm_types.h
@@ -19,6 +19,7 @@
#include <linux/workqueue.h>
#include <linux/seqlock.h>
#include <linux/percpu_counter.h>
+#include <linux/types.h>
#include <asm/mmu.h>
@@ -629,9 +630,8 @@ static inline struct anon_vma_name *anon
}
#endif
-struct vma_lock {
- struct rw_semaphore lock;
-};
+#define VMA_LOCK_OFFSET 0x40000000
+#define VMA_REF_LIMIT (VMA_LOCK_OFFSET - 1)
struct vma_numab_state {
/*
@@ -710,18 +710,12 @@ struct vm_area_struct {
#ifdef CONFIG_PER_VMA_LOCK
/*
- * Flag to indicate areas detached from the mm->mm_mt tree.
- * Unstable RCU readers are allowed to read this.
- */
- bool detached;
-
- /*
* Can only be written (using WRITE_ONCE()) while holding both:
* - mmap_lock (in write mode)
- * - vm_lock->lock (in write mode)
+ * - vm_refcnt bit at VMA_LOCK_OFFSET is set
* Can be read reliably while holding one of:
* - mmap_lock (in read or write mode)
- * - vm_lock->lock (in read or write mode)
+ * - vm_refcnt bit at VMA_LOCK_OFFSET is set or vm_refcnt > 1
* Can be read unreliably (using READ_ONCE()) for pessimistic bailout
* while holding nothing (except RCU to keep the VMA struct allocated).
*
@@ -784,7 +778,10 @@ struct vm_area_struct {
struct vm_userfaultfd_ctx vm_userfaultfd_ctx;
#ifdef CONFIG_PER_VMA_LOCK
/* Unstable RCU readers are allowed to read this. */
- struct vma_lock vm_lock ____cacheline_aligned_in_smp;
+ refcount_t vm_refcnt ____cacheline_aligned_in_smp;
+#ifdef CONFIG_DEBUG_LOCK_ALLOC
+ struct lockdep_map vmlock_dep_map;
+#endif
#endif
} __randomize_layout;
@@ -919,6 +916,7 @@ struct mm_struct {
* by mmlist_lock
*/
#ifdef CONFIG_PER_VMA_LOCK
+ struct rcuwait vma_writer_wait;
/*
* This field has lock-like semantics, meaning it is sometimes
* accessed with ACQUIRE/RELEASE semantics.
--- a/kernel/fork.c~mm-replace-vm_lock-and-detached-flag-with-a-reference-count
+++ a/kernel/fork.c
@@ -463,12 +463,8 @@ struct vm_area_struct *vm_area_dup(struc
* will be reinitialized.
*/
data_race(memcpy(new, orig, sizeof(*new)));
- vma_lock_init(new);
+ vma_lock_init(new, true);
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_numab_state_init(new);
dup_anon_vma_name(orig, new);
@@ -477,6 +473,8 @@ struct vm_area_struct *vm_area_dup(struc
void __vm_area_free(struct vm_area_struct *vma)
{
+ /* The vma should be detached while being destroyed. */
+ vma_assert_detached(vma);
vma_numab_state_free(vma);
free_anon_vma_name(vma);
kmem_cache_free(vm_area_cachep, vma);
@@ -488,8 +486,6 @@ static void vm_area_free_rcu_cb(struct r
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);
}
#endif
@@ -1223,6 +1219,9 @@ static inline void mmap_init_lock(struct
{
init_rwsem(&mm->mmap_lock);
mm_lock_seqcount_init(mm);
+#ifdef CONFIG_PER_VMA_LOCK
+ rcuwait_init(&mm->vma_writer_wait);
+#endif
}
static struct mm_struct *mm_init(struct mm_struct *mm, struct task_struct *p,
--- a/mm/init-mm.c~mm-replace-vm_lock-and-detached-flag-with-a-reference-count
+++ a/mm/init-mm.c
@@ -40,6 +40,7 @@ struct mm_struct init_mm = {
.arg_lock = __SPIN_LOCK_UNLOCKED(init_mm.arg_lock),
.mmlist = LIST_HEAD_INIT(init_mm.mmlist),
#ifdef CONFIG_PER_VMA_LOCK
+ .vma_writer_wait = __RCUWAIT_INITIALIZER(init_mm.vma_writer_wait),
.mm_lock_seq = SEQCNT_ZERO(init_mm.mm_lock_seq),
#endif
.user_ns = &init_user_ns,
--- a/mm/memory.c~mm-replace-vm_lock-and-detached-flag-with-a-reference-count
+++ a/mm/memory.c
@@ -6328,9 +6328,47 @@ fail:
#endif
#ifdef CONFIG_PER_VMA_LOCK
+static inline bool __vma_enter_locked(struct vm_area_struct *vma, bool detaching)
+{
+ unsigned int tgt_refcnt = VMA_LOCK_OFFSET;
+
+ /* Additional refcnt if the vma is attached. */
+ if (!detaching)
+ tgt_refcnt++;
+
+ /*
+ * If vma is detached then only vma_mark_attached() can raise the
+ * vm_refcnt. mmap_write_lock prevents racing with vma_mark_attached().
+ */
+ if (!refcount_add_not_zero(VMA_LOCK_OFFSET, &vma->vm_refcnt))
+ return false;
+
+ rwsem_acquire(&vma->vmlock_dep_map, 0, 0, _RET_IP_);
+ rcuwait_wait_event(&vma->vm_mm->vma_writer_wait,
+ refcount_read(&vma->vm_refcnt) == tgt_refcnt,
+ TASK_UNINTERRUPTIBLE);
+ lock_acquired(&vma->vmlock_dep_map, _RET_IP_);
+
+ return true;
+}
+
+static inline void __vma_exit_locked(struct vm_area_struct *vma, bool *detached)
+{
+ *detached = refcount_sub_and_test(VMA_LOCK_OFFSET, &vma->vm_refcnt);
+ rwsem_release(&vma->vmlock_dep_map, _RET_IP_);
+}
+
void __vma_start_write(struct vm_area_struct *vma, unsigned int mm_lock_seq)
{
- down_write(&vma->vm_lock.lock);
+ bool locked;
+
+ /*
+ * __vma_enter_locked() returns false immediately if the vma is not
+ * attached, otherwise it waits until refcnt is indicating that vma
+ * is attached with no readers.
+ */
+ locked = __vma_enter_locked(vma, false);
+
/*
* We should use WRITE_ONCE() here because we can have concurrent reads
* from the early lockless pessimistic check in vma_start_read().
@@ -6338,10 +6376,40 @@ void __vma_start_write(struct vm_area_st
* we should use WRITE_ONCE() for cleanliness and to keep KCSAN happy.
*/
WRITE_ONCE(vma->vm_lock_seq, mm_lock_seq);
- up_write(&vma->vm_lock.lock);
+
+ if (locked) {
+ bool detached;
+
+ __vma_exit_locked(vma, &detached);
+ VM_BUG_ON_VMA(detached, vma); /* vma should remain attached */
+ }
}
EXPORT_SYMBOL_GPL(__vma_start_write);
+void vma_mark_detached(struct vm_area_struct *vma)
+{
+ vma_assert_write_locked(vma);
+ vma_assert_attached(vma);
+
+ /*
+ * We are the only writer, so no need to use vma_refcount_put().
+ * The condition below is unlikely because the vma has been already
+ * write-locked and readers can increment vm_refcnt only temporarily
+ * before they check vm_lock_seq, realize the vma is locked and drop
+ * back the vm_refcnt. That is a narrow window for observing a raised
+ * vm_refcnt.
+ */
+ if (unlikely(!refcount_dec_and_test(&vma->vm_refcnt))) {
+ /* Wait until vma is detached with no readers. */
+ if (__vma_enter_locked(vma, true)) {
+ bool detached;
+
+ __vma_exit_locked(vma, &detached);
+ VM_BUG_ON_VMA(!detached, vma);
+ }
+ }
+}
+
/*
* Lookup and lock a VMA under RCU protection. Returned VMA is guaranteed to be
* stable and not isolated. If the VMA is not found or is being modified the
@@ -6354,7 +6422,6 @@ struct vm_area_struct *lock_vma_under_rc
struct vm_area_struct *vma;
rcu_read_lock();
-retry:
vma = mas_walk(&mas);
if (!vma)
goto inval;
@@ -6362,13 +6429,6 @@ retry:
if (!vma_start_read(vma))
goto inval;
- /* Check if the VMA got isolated after we found it */
- if (is_vma_detached(vma)) {
- vma_end_read(vma);
- count_vm_vma_lock_event(VMA_LOCK_MISS);
- /* The area was replaced with another one */
- goto retry;
- }
/*
* At this point, we have a stable reference to a VMA: The VMA is
* locked and we know it hasn't already been isolated.
--- a/tools/testing/vma/linux/atomic.h~mm-replace-vm_lock-and-detached-flag-with-a-reference-count
+++ a/tools/testing/vma/linux/atomic.h
@@ -9,4 +9,9 @@
#define atomic_set(x, y) uatomic_set(x, y)
#define U8_MAX UCHAR_MAX
+#ifndef atomic_cmpxchg_relaxed
+#define atomic_cmpxchg_relaxed uatomic_cmpxchg
+#define atomic_cmpxchg_release uatomic_cmpxchg
+#endif /* atomic_cmpxchg_relaxed */
+
#endif /* _LINUX_ATOMIC_H */
--- a/tools/testing/vma/vma_internal.h~mm-replace-vm_lock-and-detached-flag-with-a-reference-count
+++ a/tools/testing/vma/vma_internal.h
@@ -25,7 +25,7 @@
#include <linux/maple_tree.h>
#include <linux/mm.h>
#include <linux/rbtree.h>
-#include <linux/rwsem.h>
+#include <linux/refcount.h>
extern unsigned long stack_guard_gap;
#ifdef CONFIG_MMU
@@ -132,10 +132,6 @@ typedef __bitwise unsigned int vm_fault_
*/
#define pr_warn_once pr_err
-typedef struct refcount_struct {
- atomic_t refs;
-} refcount_t;
-
struct kref {
refcount_t refcount;
};
@@ -228,15 +224,12 @@ struct mm_struct {
unsigned long def_flags;
};
-struct vma_lock {
- struct rw_semaphore lock;
-};
-
-
struct file {
struct address_space *f_mapping;
};
+#define VMA_LOCK_OFFSET 0x40000000
+
struct vm_area_struct {
/* The first cache line has the info for VMA tree walking. */
@@ -264,16 +257,13 @@ struct vm_area_struct {
};
#ifdef CONFIG_PER_VMA_LOCK
- /* Flag to indicate areas detached from the mm->mm_mt tree */
- bool detached;
-
/*
* Can only be written (using WRITE_ONCE()) while holding both:
* - mmap_lock (in write mode)
- * - vm_lock.lock (in write mode)
+ * - vm_refcnt bit at VMA_LOCK_OFFSET is set
* Can be read reliably while holding one of:
* - mmap_lock (in read or write mode)
- * - vm_lock.lock (in read or write mode)
+ * - vm_refcnt bit at VMA_LOCK_OFFSET is set or vm_refcnt > 1
* Can be read unreliably (using READ_ONCE()) for pessimistic bailout
* while holding nothing (except RCU to keep the VMA struct allocated).
*
@@ -282,7 +272,6 @@ struct vm_area_struct {
* slowpath.
*/
unsigned int vm_lock_seq;
- struct vma_lock vm_lock;
#endif
/*
@@ -335,6 +324,10 @@ struct vm_area_struct {
struct vma_numab_state *numab_state; /* NUMA Balancing state */
#endif
struct vm_userfaultfd_ctx vm_userfaultfd_ctx;
+#ifdef CONFIG_PER_VMA_LOCK
+ /* Unstable RCU readers are allowed to read this. */
+ refcount_t vm_refcnt;
+#endif
} __randomize_layout;
struct vm_fault {};
@@ -459,23 +452,41 @@ static inline struct vm_area_struct *vma
return mas_find(&vmi->mas, ULONG_MAX);
}
-static inline void vma_lock_init(struct vm_area_struct *vma)
+/*
+ * WARNING: to avoid racing with vma_mark_attached()/vma_mark_detached(), these
+ * assertions should be made either under mmap_write_lock or when the object
+ * has been isolated under mmap_write_lock, ensuring no competing writers.
+ */
+static inline void vma_assert_attached(struct vm_area_struct *vma)
+{
+ VM_BUG_ON_VMA(!refcount_read(&vma->vm_refcnt), vma);
+}
+
+static inline void vma_assert_detached(struct vm_area_struct *vma)
{
- init_rwsem(&vma->vm_lock.lock);
- vma->vm_lock_seq = UINT_MAX;
+ VM_BUG_ON_VMA(refcount_read(&vma->vm_refcnt), vma);
}
+static inline void vma_assert_write_locked(struct vm_area_struct *);
static inline void vma_mark_attached(struct vm_area_struct *vma)
{
- vma->detached = false;
+ vma_assert_write_locked(vma);
+ vma_assert_detached(vma);
+ refcount_set(&vma->vm_refcnt, 1);
}
-static inline void vma_assert_write_locked(struct vm_area_struct *);
static inline void vma_mark_detached(struct vm_area_struct *vma)
{
- /* When detaching vma should be write-locked */
vma_assert_write_locked(vma);
- vma->detached = true;
+ vma_assert_attached(vma);
+
+ /* We are the only writer, so no need to use vma_refcount_put(). */
+ if (unlikely(!refcount_dec_and_test(&vma->vm_refcnt))) {
+ /*
+ * Reader must have temporarily raised vm_refcnt but it will
+ * drop it without using the vma since vma is write-locked.
+ */
+ }
}
extern const struct vm_operations_struct vma_dummy_vm_ops;
@@ -488,9 +499,7 @@ static inline void vma_init(struct vm_ar
vma->vm_mm = mm;
vma->vm_ops = &vma_dummy_vm_ops;
INIT_LIST_HEAD(&vma->anon_vma_chain);
- /* vma is not locked, can't use vma_mark_detached() */
- vma->detached = true;
- vma_lock_init(vma);
+ vma->vm_lock_seq = UINT_MAX;
}
static inline struct vm_area_struct *vm_area_alloc(struct mm_struct *mm)
@@ -513,10 +522,9 @@ static inline struct vm_area_struct *vm_
return NULL;
memcpy(new, orig, sizeof(*new));
- vma_lock_init(new);
+ refcount_set(&new->vm_refcnt, 0);
+ new->vm_lock_seq = UINT_MAX;
INIT_LIST_HEAD(&new->anon_vma_chain);
- /* vma is not locked, can't use vma_mark_detached() */
- new->detached = true;
return new;
}
_
Patches currently in -mm which might be from surenb@xxxxxxxxxx are
alloc_tag-skip-pgalloc_tag_swap-if-profiling-is-disabled.patch
tools-fix-atomic_set-definition-to-set-the-value-correctly.patch
seqlock-add-raw_seqcount_try_begin.patch
mm-convert-mm_lock_seq-to-a-proper-seqcount.patch
mm-introduce-mmap_lock_speculate_try_beginretry.patch
mm-introduce-vma_start_read_locked_nested-helpers.patch
mm-move-per-vma-lock-into-vm_area_struct.patch
mm-mark-vma-as-detached-until-its-added-into-vma-tree.patch
mm-introduce-vma_iter_store_attached-to-use-with-attached-vmas.patch
mm-mark-vmas-detached-upon-exit.patch
types-move-struct-rcuwait-into-typesh.patch
mm-allow-vma_start_read_locked-vma_start_read_locked_nested-to-fail.patch
mm-move-mmap_init_lock-out-of-the-header-file.patch
mm-uninline-the-main-body-of-vma_start_write.patch
refcount-introduce-__refcount_addinc_not_zero_limited.patch
mm-replace-vm_lock-and-detached-flag-with-a-reference-count.patch
mm-move-lesser-used-vma_area_struct-members-into-the-last-cacheline.patch
mm-debug-print-vm_refcnt-state-when-dumping-the-vma.patch
mm-remove-extra-vma_numab_state_init-call.patch
mm-prepare-lock_vma_under_rcu-for-vma-reuse-possibility.patch
mm-make-vma-cache-slab_typesafe_by_rcu.patch
docs-mm-document-latest-changes-to-vm_lock.patch
alloc_tag-avoid-current-alloc_tag-manipulations-when-profiling-is-disabled.patch
