Re: [PATCH v7 12/17] mm: replace vm_lock and detached flag with a reference count

Liam R. Howlett
2025-01-07T18:44:57Z

* Suren Baghdasaryan <surenb@google.com> [241226 12:07]:
> 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 both increments vm_refcnt and
> 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.

I find the above a bit hard to parse.

What I understand is:
1. all accesses increment the ref count.
2. readers cannot increment the ref count unless the writer bit is 0 (no
write present)
3. writers must wait for the ref count to reach 2 (the tree + writer
reference) before proceeding.
4. increment overflow must be handled by the readers.

>
> Suggested-by: Peter Zijlstra <peterz@infradead.org>
> Suggested-by: Matthew Wilcox <willy@infradead.org>
> Signed-off-by: Suren Baghdasaryan <surenb@google.com>
> ---
> include/linux/mm.h | 100 +++++++++++++++++++++----------
> include/linux/mm_types.h | 22 ++++---
> kernel/fork.c | 13 ++--
> mm/init-mm.c | 1 +
> mm/memory.c | 68 +++++++++++++++++----
> tools/testing/vma/linux/atomic.h | 5 ++
> tools/testing/vma/vma_internal.h | 66 +++++++++++---------
> 7 files changed, 185 insertions(+), 90 deletions(-)
>
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index ea4c4228b125..99f4720d7e51 100644
> --- a/include/linux/mm.h
> +++ b/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 +698,34 @@ static inline void vma_numab_state_free(struct vm_area_struct *vma) {}
> #endif /* CONFIG_NUMA_BALANCING */
>
> #ifdef CONFIG_PER_VMA_LOCK
> -static inline void vma_lock_init(struct vm_area_struct *vma)
> +static inline void vma_lockdep_init(struct vm_area_struct *vma)
> {
> - 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
> +}
> +
> +static inline void vma_init_lock(struct vm_area_struct *vma, bool reset_refcnt)
> +{
> + if (reset_refcnt)
> + refcount_set(&vma->vm_refcnt, 0);
> vma->vm_lock_seq = UINT_MAX;
> }
>
> +static inline void vma_refcount_put(struct vm_area_struct *vma)
> +{
> + int refcnt;
> +
> + if (!__refcount_dec_and_test(&vma->vm_refcnt, &refcnt)) {
> + rwsem_release(&vma->vmlock_dep_map, _RET_IP_);
> +
> + if (refcnt & VMA_LOCK_OFFSET)

Couldn't we only wake on refcnt == VMA_LOCK_OFFSET + 2?
Right now you will wake on every departed reader, I think? We know
refcnt is only going down if VMA_LOCK_OFFSET is set.

Also, maybe a #define for VMA_LOCK_WRITER_ONLY or some better name?

> + rcuwait_wake_up(&vma->vm_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 +733,8 @@ static inline void vma_lock_init(struct vm_area_struct *vma)
> */
> 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 +745,20 @@ static inline bool vma_start_read(struct vm_area_struct *vma)
> 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))
> +
> + rwsem_acquire_read(&vma->vmlock_dep_map, 0, 0, _RET_IP_);
> + /* Limit at VMA_REF_LIMIT to leave one count for a writer */
> + if (unlikely(!__refcount_inc_not_zero_limited(&vma->vm_refcnt, &oldcnt,
> + VMA_REF_LIMIT))) {
> + rwsem_release(&vma->vmlock_dep_map, _RET_IP_);
> return false;
> + }
> + lock_acquired(&vma->vmlock_dep_map, _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
> @@ -734,10 +766,12 @@ 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))) {
> - up_read(&vma->vm_lock.lock);
> + if (unlikely(oldcnt & VMA_LOCK_OFFSET ||
> + vma->vm_lock_seq == raw_read_seqcount(&vma->vm_mm->mm_lock_seq))) {
> + vma_refcount_put(vma);
> return false;
> }
> +
> return true;
> }
>
> @@ -749,8 +783,17 @@ static inline bool vma_start_read(struct vm_area_struct *vma)
> */
> 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);
> + rwsem_acquire_read(&vma->vmlock_dep_map, subclass, 0, _RET_IP_);
> + /* Limit at VMA_REF_LIMIT to leave one count for a writer */
> + if (unlikely(!__refcount_inc_not_zero_limited(&vma->vm_refcnt, &oldcnt,
> + VMA_REF_LIMIT))) {
> + rwsem_release(&vma->vmlock_dep_map, _RET_IP_);
> + return false;
> + }
> + lock_acquired(&vma->vmlock_dep_map, _RET_IP_);
> return true;
> }
>
> @@ -762,15 +805,13 @@ static inline bool vma_start_read_locked_nested(struct vm_area_struct *vma, int
> */
> 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);
> + vma_refcount_put(vma);
> rcu_read_unlock();
> }
>
> @@ -813,36 +854,33 @@ static inline void vma_assert_write_locked(struct vm_area_struct *vma)
>
> 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 +903,8 @@ 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 void vma_lockdep_init(struct vm_area_struct *vma) {}
> +static inline void vma_init_lock(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 +947,9 @@ static inline void vma_init(struct vm_area_struct *vma, struct mm_struct *mm)
> 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_lockdep_init(vma);
> + vma_init_lock(vma, false);
> }
>
> /* 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 6573d95f1d1e..b5312421dec6 100644
> --- a/include/linux/mm_types.h
> +++ b/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_vma_name_alloc(const char *name)
> }
> #endif
>
> -struct vma_lock {
> - struct rw_semaphore lock;
> -};
> +#define VMA_LOCK_OFFSET 0x40000000
> +#define VMA_REF_LIMIT (VMA_LOCK_OFFSET - 2)
>
> struct vma_numab_state {
> /*
> @@ -709,19 +709,13 @@ 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.
> diff --git a/kernel/fork.c b/kernel/fork.c
> index d4c75428ccaf..7a0800d48112 100644
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -463,12 +463,8 @@ struct vm_area_struct *vm_area_dup(struct vm_area_struct *orig)
> * will be reinitialized.
> */
> data_race(memcpy(new, orig, sizeof(*new)));
> - vma_lock_init(new);
> + vma_init_lock(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(struct vm_area_struct *orig)
>
> 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 rcu_head *head)
> 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 mm_struct *mm)
> {
> 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,
> diff --git a/mm/init-mm.c b/mm/init-mm.c
> index 6af3ad675930..4600e7605cab 100644
> --- a/mm/init-mm.c
> +++ b/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,
> diff --git a/mm/memory.c b/mm/memory.c
> index 236fdecd44d6..2def47b5dff0 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -6328,9 +6328,39 @@ struct vm_area_struct *lock_mm_and_find_vma(struct mm_struct *mm,
> #endif
>
> #ifdef CONFIG_PER_VMA_LOCK
> +static inline bool __vma_enter_locked(struct vm_area_struct *vma, unsigned int 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_inc_not_zero(&vma->vm_refcnt))
> + return false;

Can't the write lock overflow the ref count too?

> +
> + rwsem_acquire(&vma->vmlock_dep_map, 0, 0, _RET_IP_);
> + /* vma is attached, set the writer present bit */
> + refcount_add(VMA_LOCK_OFFSET, &vma->vm_refcnt);
> + 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 + 1, &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;
> +
> + /* Wait until refcnt is (VMA_LOCK_OFFSET + 2) => attached with no readers */
> + locked = __vma_enter_locked(vma, VMA_LOCK_OFFSET + 2);

Does it need to take a ref count at all? Could we just set the write
bit and wait for it to become 1 instead? That is, 1 would represent
detached or writer is about to attach/detach it.

If we do need it to be ref counted for the writer, we could set the
write bit and the wait for the ref to be 1 before incrementing it to 2?
I think this would be safer as we know there is only one writer and the
readers can only decrease after setting the write bit.

> +
> /*
> * We should use WRITE_ONCE() here because we can have concurrent reads
> * from the early lockless pessimistic check in vma_start_read().
> @@ -6338,10 +6368,36 @@ void __vma_start_write(struct vm_area_struct *vma, unsigned int mm_lock_seq)
> * 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(). */
> + if (unlikely(!refcount_dec_and_test(&vma->vm_refcnt))) {
> + /*
> + * Wait until refcnt is (VMA_LOCK_OFFSET + 1) => detached with
> + * no readers
> + */
> + if (__vma_enter_locked(vma, VMA_LOCK_OFFSET + 1)) {
> + 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 +6410,6 @@ struct vm_area_struct *lock_vma_under_rcu(struct mm_struct *mm,
> struct vm_area_struct *vma;
>
> rcu_read_lock();
> -retry:
> vma = mas_walk(&mas);
> if (!vma)
> goto inval;
> @@ -6362,13 +6417,6 @@ struct vm_area_struct *lock_vma_under_rcu(struct mm_struct *mm,
> 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.
> diff --git a/tools/testing/vma/linux/atomic.h b/tools/testing/vma/linux/atomic.h
> index e01f66f98982..2e2021553196 100644
> --- a/tools/testing/vma/linux/atomic.h
> +++ b/tools/testing/vma/linux/atomic.h
> @@ -9,4 +9,9 @@
> #define atomic_set(x, y) do {} while (0)
> #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 */
> diff --git a/tools/testing/vma/vma_internal.h b/tools/testing/vma/vma_internal.h
> index 2a624f9304da..1e8cd2f013fa 100644
> --- a/tools/testing/vma/vma_internal.h
> +++ b/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_t;
> */
> #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_next(struct vma_iterator *vmi)
> 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)
> {
> - 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_mark_attached(struct vm_area_struct *vma)
> +static inline void vma_assert_detached(struct vm_area_struct *vma)
> {
> - vma->detached = false;
> + 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_assert_write_locked(vma);
> + vma_assert_detached(vma);
> + refcount_set(&vma->vm_refcnt, 1);
> +}
> +
> 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_area_struct *vma, struct mm_struct *mm)
> 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_area_dup(struct vm_area_struct *orig)
> 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;
> }
> --
> 2.47.1.613.gc27f4b7a9f-goog
>