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Message-ID: <CAJuCfpE_odESLmQ5j2318JNu6XfvJJX=7jsy5CEJy02M2U2aqw@mail.gmail.com>
Date: Tue, 7 Jan 2025 11:38:38 -0800
From: Suren Baghdasaryan <surenb@...gle.com>
To: "Liam R. Howlett" <Liam.Howlett@...cle.com>, Suren Baghdasaryan <surenb@...gle.com>,
akpm@...ux-foundation.org, peterz@...radead.org, willy@...radead.org,
lorenzo.stoakes@...cle.com, mhocko@...e.com, vbabka@...e.cz,
hannes@...xchg.org, mjguzik@...il.com, oliver.sang@...el.com,
mgorman@...hsingularity.net, david@...hat.com, peterx@...hat.com,
oleg@...hat.com, dave@...olabs.net, paulmck@...nel.org, brauner@...nel.org,
dhowells@...hat.com, hdanton@...a.com, hughd@...gle.com,
lokeshgidra@...gle.com, minchan@...gle.com, jannh@...gle.com,
shakeel.butt@...ux.dev, souravpanda@...gle.com, pasha.tatashin@...een.com,
klarasmodin@...il.com, corbet@....net, linux-doc@...r.kernel.org,
linux-mm@...ck.org, linux-kernel@...r.kernel.org, kernel-team@...roid.com
Subject: Re: [PATCH v7 12/17] mm: replace vm_lock and detached flag with a
reference count
On Tue, Jan 7, 2025 at 10:44 AM Liam R. Howlett <Liam.Howlett@...cle.com> wrote:
>
> * Suren Baghdasaryan <surenb@...gle.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.
Your understanding is correct. I will add this summary to the
description to make it more understandable.
>
> >
> > Suggested-by: Peter Zijlstra <peterz@...radead.org>
> > Suggested-by: Matthew Wilcox <willy@...radead.org>
> > Signed-off-by: Suren Baghdasaryan <surenb@...gle.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.
I think we can, except vma_mark_detached() can be waiting for readers
after detaching the vma, so the refcount value it's waiting for is
VMA_LOCK_OFFSET + 1 (detached vma with no readers).
I think if we change the condition to (refcnt & VMA_LOCK_OFFSET) &&
(refcnt <= VMA_LOCK_OFFSET + 2) that would work. We might occasionally
wake a writer unnecessarily if it's waiting inside vma_mark_detached()
for VMA_LOCK_OFFSET + 1 but this situation is rare (see my later
comment).
>
> Also, maybe a #define for VMA_LOCK_WRITER_ONLY or some better name?
Perhaps a function would be better?
static inline bool is_vma_writer_only(refcnt)
{
return (refcnt & VMA_LOCK_OFFSET) && (refcnt <= VMA_LOCK_OFFSET + 2);
}
>
>
> > + 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?
No. VMA_REF_LIMIT is VMA_LOCK_OFFSET - 2 with one count reserved for a
possible writer (see the comment in vma_start_read()) and there can be
only one writer at a time. So, readers can raise the ref count up to
VMA_LOCK_OFFSET - 2 and a writer can raise it one more to
VMA_LOCK_OFFSET - 1 but not higher.
>
> > +
> > + 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.
Yeah, I think you are right. We can use VMA_LOCK_OFFSET alone without
taking the ref count for the writer. Something like this:
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_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_);
}
I'll try that and see if we missed anything.
>
> 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(). */
I should probably add a comment explaining why the below condition is
very unlikely (readers can increment vm_refcnt only temporarily before
they realize the vma is locked and drop the vm_refcnt back. That is a
very narrow window).
> > + 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
> >
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