mm: add per-VMA lock and helper functions to control it

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).

Link: https://lkml.kernel.org/r/20230227173632.3292573-13-surenb@google.com
Signed-off-by: Suren Baghdasaryan <surenb@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 55d0235..53bfb42 100644 (file)
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -624,6 +624,87 @@ 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 = {};
@@ -632,6 +713,7 @@ 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 f203e25..843a458 100644 (file)
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -503,6 +503,11 @@ 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.
@@ -639,6 +644,9 @@ 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 e49ba91..aab8f1b 100644 (file)
--- 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 93ec6e1..0907776 100644 (file)
--- 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;
@@ -1208,6 +1209,9 @@ 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 c9327ab..3326931 100644 (file)
--- 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