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>

1 files changed, 4 insertions, 0 deletions

diff --git a/kernel/fork.c b/kernel/fork.c
index 93ec6e14bb65e..0907776d8ed43 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;
@@ -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;