[PATCH 2/3] mm: change vma_start_read to fail to lock a detached VMA

[Suren Baghdasaryan <surenb@google.com>]

vma_start_read can successfully lock a detached VMA relying on its
caller to check for vma->detached. Change vma_start_read to include that
check and fail if the VMA got detached.

Suggested-by: Linus Torvalds <torvalds@linuxfoundation.org>
Signed-off-by: Suren Baghdasaryan <surenb@google.com>
---
 include/linux/mm.h            |  4 ++--
 include/linux/vm_event_item.h |  1 -
 mm/memory.c                   | 10 +---------
 mm/vmstat.c                   |  1 -
 4 files changed, 3 insertions(+), 13 deletions(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 8410da79c570..74e3033c9fc2 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -651,8 +651,8 @@ static inline bool vma_start_read(struct vm_area_struct *vma)
 	if (unlikely(down_read_trylock(&vma->vm_lock->lock) == 0))
 		return false;
 
-	/* Fail if VMA was write-locked after we checked it earlier */
-	if (unlikely(vm_lock_seq != READ_ONCE(vma->vm_lock_seq))) {
+	/* Fail if VMA is detached or was write-locked after we checked it earlier */
+	if (unlikely(vma->detached || vm_lock_seq != READ_ONCE(vma->vm_lock_seq))) {
 		up_read(&vma->vm_lock->lock);
 		return false;
 	}
diff --git a/include/linux/vm_event_item.h b/include/linux/vm_event_item.h
index 8abfa1240040..e741098954ff 100644
--- a/include/linux/vm_event_item.h
+++ b/include/linux/vm_event_item.h
@@ -154,7 +154,6 @@ enum vm_event_item { PGPGIN, PGPGOUT, PSWPIN, PSWPOUT,
 		VMA_LOCK_SUCCESS,
 		VMA_LOCK_ABORT,
 		VMA_LOCK_RETRY,
-		VMA_LOCK_MISS,
 #endif
 		NR_VM_EVENT_ITEMS
 };
diff --git a/mm/memory.c b/mm/memory.c
index f69fbc251198..bd666fa20748 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -5275,7 +5275,7 @@ 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;
@@ -5306,14 +5306,6 @@ struct vm_area_struct *lock_vma_under_rcu(struct mm_struct *mm,
 		goto inval;
 	}
 
-	/* Check if the VMA got isolated after we found it */
-	if (vma->detached) {
-		vma_end_read(vma);
-		count_vm_vma_lock_event(VMA_LOCK_MISS);
-		/* The area was replaced with another one */
-		goto retry;
-	}
-
 	rcu_read_unlock();
 	return vma;
 inval:
diff --git a/mm/vmstat.c b/mm/vmstat.c
index c28046371b45..5d6acdb1b611 100644
--- a/mm/vmstat.c
+++ b/mm/vmstat.c
@@ -1403,7 +1403,6 @@ const char * const vmstat_text[] = {
 	"vma_lock_success",
 	"vma_lock_abort",
 	"vma_lock_retry",
-	"vma_lock_miss",
 #endif
 #endif /* CONFIG_VM_EVENT_COUNTERS || CONFIG_MEMCG */
 };
-- 
2.41.0.162.gfafddb0af9-goog