[RFC PATCH 21/28] mm: introduce find_and_lock_anon_vma to be used from arch-specific code

[Suren Baghdasaryan]

Introduce find_and_lock_anon_vma function to lookup and lock an anonymous
VMA during page fault handling. When VMA is not found, can't be locked
or changes after being locked, the function returns NULL. The lookup is
performed under RCU protection to prevent the found VMA from being
destroyed before the VMA lock is acquired. VMA lock statistics are
updated according to the results.

Signed-off-by: Suren Baghdasaryan <surenb at google.com>
---
 include/linux/mm.h |  3 +++
 mm/memory.c        | 45 +++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 48 insertions(+)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 7c3190eaabd7..a3cbaa7b9119 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -684,6 +684,9 @@ static inline void vma_assert_no_reader(struct vm_area_struct *vma)
 		      vma);
 }
 
+struct vm_area_struct *find_and_lock_anon_vma(struct mm_struct *mm,
+					      unsigned long address);
+
 #else /* CONFIG_PER_VMA_LOCK */
 
 static inline void vma_init_lock(struct vm_area_struct *vma) {}
diff --git a/mm/memory.c b/mm/memory.c
index 29d2f49f922a..bf557f7056de 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -5183,6 +5183,51 @@ vm_fault_t handle_mm_fault(struct vm_area_struct *vma, unsigned long address,
 }
 EXPORT_SYMBOL_GPL(handle_mm_fault);
 
+#ifdef CONFIG_PER_VMA_LOCK
+static inline struct vm_area_struct *find_vma_under_rcu(struct mm_struct *mm,
+							unsigned long address)
+{
+	struct vm_area_struct *vma = __find_vma(mm, address);
+
+	if (!vma || vma->vm_start > address)
+		return NULL;
+
+	if (!vma_is_anonymous(vma))
+		return NULL;
+
+	if (!vma_read_trylock(vma)) {
+		count_vm_vma_lock_event(VMA_LOCK_ABORT);
+		return NULL;
+	}
+
+	/* Check if the VMA got isolated after we found it */
+	if (RB_EMPTY_NODE(&vma->vm_rb)) {
+		vma_read_unlock(vma);
+		count_vm_vma_lock_event(VMA_LOCK_MISS);
+		return NULL;
+	}
+
+	return vma;
+}
+
+/*
+ * Lookup and lock and anonymous VMA. Returned VMA is guaranteed to be stable
+ * and not isolated. If the VMA is not found of is being modified the function
+ * returns NULL.
+ */
+struct vm_area_struct *find_and_lock_anon_vma(struct mm_struct *mm,
+					      unsigned long address)
+{
+	struct vm_area_struct *vma;
+
+	rcu_read_lock();
+	vma = find_vma_under_rcu(mm, address);
+	rcu_read_unlock();
+
+	return vma;
+}
+#endif /* CONFIG_PER_VMA_LOCK */
+
 #ifndef __PAGETABLE_P4D_FOLDED
 /*
  * Allocate p4d page table.
-- 
2.37.2.672.g94769d06f0-goog