Introduce per-VMA locking. The lock implementation relies on a
per-vma and per-mm sequence counters to note exclusive locking:
- read lock - (implemented by vma_start_read) requires the vma
(vm_lock_seq) and mm (mm_lock_seq) sequence counters to differ.
If they match then there must be a vma exclusive lock held somewhere.
- read unlock - (implemented by vma_end_read) is a trivial vma->lock
unlock.
- write lock - (vma_start_write) requires the mmap_lock to be held
exclusively and the current mm counter is assigned to the vma counter.
This will allow multiple vmas to be locked under a single mmap_lock
write lock (e.g. during vma merging). The vma counter is modified
under exclusive vma lock.
- write unlock - (vma_end_write_all) is a batch release of all vma
locks held. It doesn't pair with a specific vma_start_write! It is
done before exclusive mmap_lock is released by incrementing mm
sequence counter (mm_lock_seq).
- write downgrade - if the mmap_lock is downgraded to the read lock, all
vma write locks are released as well (effectivelly same as write
unlock).
Signed-off-by: Suren Baghdasaryan <surenb@xxxxxxxxxx>
---
include/linux/mm.h | 82 +++++++++++++++++++++++++++++++++++++++
include/linux/mm_types.h | 8 ++++
include/linux/mmap_lock.h | 13 +++++++
kernel/fork.c | 4 ++
mm/init-mm.c | 3 ++
5 files changed, 110 insertions(+)
diff --git a/include/linux/mm.h b/include/linux/mm.h
index dd295c020e85..fee08e8fdce7 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -617,6 +617,87 @@ struct vm_operations_struct {
unsigned long addr);
};
+#ifdef CONFIG_PER_VMA_LOCK
+static inline void vma_init_lock(struct vm_area_struct *vma)
+{
+ init_rwsem(&vma->lock);
+ vma->vm_lock_seq = -1;
+}
+
+/*
+ * 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)
+{
+ /* Check before locking. A race might cause false locked result. */
+ if (vma->vm_lock_seq == READ_ONCE(vma->vm_mm->mm_lock_seq))
+ return false;
+
+ if (unlikely(down_read_trylock(&vma->lock) == 0))
+ return false;
+
+ /*
+ * Overflow might produce false locked result.
+ * False unlocked result is impossible because we modify and check
+ * vma->vm_lock_seq under vma->lock protection and mm->mm_lock_seq
+ * modification invalidates all existing locks.
+ */
+ if (unlikely(vma->vm_lock_seq == READ_ONCE(vma->vm_mm->mm_lock_seq))) {
+ up_read(&vma->lock);
+ return false;
+ }
+ return true;
+}
+
+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->lock);
+ rcu_read_unlock();
+}
+
+static inline void vma_start_write(struct vm_area_struct *vma)
+{
+ int mm_lock_seq;
+
+ mmap_assert_write_locked(vma->vm_mm);
+
+ /*
+ * current task is holding mmap_write_lock, both vma->vm_lock_seq and
+ * mm->mm_lock_seq can't be concurrently modified.
+ */
+ mm_lock_seq = READ_ONCE(vma->vm_mm->mm_lock_seq);
+ if (vma->vm_lock_seq == mm_lock_seq)
+ return;
+
+ down_write(&vma->lock);
+ vma->vm_lock_seq = mm_lock_seq;
+ up_write(&vma->lock);
+}
+
+static inline void vma_assert_write_locked(struct vm_area_struct *vma)
+{
+ mmap_assert_write_locked(vma->vm_mm);
+ /*
+ * current task is holding mmap_write_lock, both vma->vm_lock_seq and
+ * mm->mm_lock_seq can't be concurrently modified.
+ */
+ VM_BUG_ON_VMA(vma->vm_lock_seq != READ_ONCE(vma->vm_mm->mm_lock_seq), vma);
+}
+
+#else /* CONFIG_PER_VMA_LOCK */
+
+static inline void vma_init_lock(struct vm_area_struct *vma) {}
+static inline bool vma_start_read(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) {}
+static inline void vma_assert_write_locked(struct vm_area_struct *vma) {}
+
+#endif /* CONFIG_PER_VMA_LOCK */
+
static inline void vma_init(struct vm_area_struct *vma, struct mm_struct *mm)
{
static const struct vm_operations_struct dummy_vm_ops = {};
@@ -625,6 +706,7 @@ static inline void vma_init(struct vm_area_struct *vma, struct mm_struct *mm)
vma->vm_mm = mm;
vma->vm_ops = &dummy_vm_ops;
INIT_LIST_HEAD(&vma->anon_vma_chain);
+ vma_init_lock(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 62e413f84011..88619c6a29a3 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -508,6 +508,11 @@ struct vm_area_struct {
vm_flags_t __private __vm_flags;
};
+#ifdef CONFIG_PER_VMA_LOCK
+ int vm_lock_seq;
+ struct rw_semaphore lock;
+#endif
+
/*
* For areas with an address space and backing store,
* linkage into the address_space->i_mmap interval tree.
@@ -633,6 +638,9 @@ struct mm_struct {
* init_mm.mmlist, and are protected
* by mmlist_lock
*/
+#ifdef CONFIG_PER_VMA_LOCK
+ int mm_lock_seq;
+#endif
unsigned long hiwater_rss; /* High-watermark of RSS usage */
diff --git a/include/linux/mmap_lock.h b/include/linux/mmap_lock.h
index e49ba91bb1f0..aab8f1b28d26 100644
--- a/include/linux/mmap_lock.h
+++ b/include/linux/mmap_lock.h
@@ -72,6 +72,17 @@ static inline void mmap_assert_write_locked(struct mm_struct *mm)
VM_BUG_ON_MM(!rwsem_is_locked(&mm->mmap_lock), mm);
}
+#ifdef CONFIG_PER_VMA_LOCK
+static inline void vma_end_write_all(struct mm_struct *mm)
+{
+ mmap_assert_write_locked(mm);
+ /* No races during update due to exclusive mmap_lock being held */
+ WRITE_ONCE(mm->mm_lock_seq, mm->mm_lock_seq + 1);
+}
+#else
+static inline void vma_end_write_all(struct mm_struct *mm) {}
+#endif
+
static inline void mmap_init_lock(struct mm_struct *mm)
{
init_rwsem(&mm->mmap_lock);
@@ -114,12 +125,14 @@ static inline bool mmap_write_trylock(struct mm_struct *mm)
static inline void mmap_write_unlock(struct mm_struct *mm)
{
__mmap_lock_trace_released(mm, true);
+ vma_end_write_all(mm);
up_write(&mm->mmap_lock);
}
static inline void mmap_write_downgrade(struct mm_struct *mm)
{
__mmap_lock_trace_acquire_returned(mm, false, true);
+ vma_end_write_all(mm);
downgrade_write(&mm->mmap_lock);
}
diff --git a/kernel/fork.c b/kernel/fork.c
index 314d51eb91da..9141427a98b2 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -474,6 +474,7 @@ struct vm_area_struct *vm_area_dup(struct vm_area_struct *orig)
*/
data_race(memcpy(new, orig, sizeof(*new)));
INIT_LIST_HEAD(&new->anon_vma_chain);
+ vma_init_lock(new);
dup_anon_vma_name(orig, new);
}
return new;
@@ -1147,6 +1148,9 @@ static struct mm_struct *mm_init(struct mm_struct *mm, struct task_struct *p,
seqcount_init(&mm->write_protect_seq);
mmap_init_lock(mm);
INIT_LIST_HEAD(&mm->mmlist);
+#ifdef CONFIG_PER_VMA_LOCK
+ mm->mm_lock_seq = 0;
+#endif
mm_pgtables_bytes_init(mm);
mm->map_count = 0;
mm->locked_vm = 0;
diff --git a/mm/init-mm.c b/mm/init-mm.c
index c9327abb771c..33269314e060 100644
--- a/mm/init-mm.c
+++ b/mm/init-mm.c
@@ -37,6 +37,9 @@ struct mm_struct init_mm = {
.page_table_lock = __SPIN_LOCK_UNLOCKED(init_mm.page_table_lock),
.arg_lock = __SPIN_LOCK_UNLOCKED(init_mm.arg_lock),
.mmlist = LIST_HEAD_INIT(init_mm.mmlist),
+#ifdef CONFIG_PER_VMA_LOCK
+ .mm_lock_seq = 0,
+#endif
.user_ns = &init_user_ns,
.cpu_bitmap = CPU_BITS_NONE,
#ifdef CONFIG_IOMMU_SVA
--
2.39.1
