mm: introduce lock_vma_under_rcu to be used from arch-specific code

Introduce lock_vma_under_rcu function to lookup and lock a 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.  For now only anonymous VMAs can be searched this way.  In other
cases the function returns NULL.

Link: https://lkml.kernel.org/r/20230227173632.3292573-24-surenb@google.com
Signed-off-by: Suren Baghdasaryan <surenb@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

include/linux/mm.h

@@ -719,6 +719,9 @@ static inline void vma_mark_detached(struct vm_area_struct *vma, bool detached)
 	vma->detached = detached;
 }
 
+struct vm_area_struct *lock_vma_under_rcu(struct mm_struct *mm,
+					  unsigned long address);
+
 #else /* CONFIG_PER_VMA_LOCK */
 
 static inline void vma_init_lock(struct vm_area_struct *vma) {}

mm/memory.c

@@ -5232,6 +5232,52 @@ 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
+/*
+ * Lookup and lock a VMA under RCU protection. Returned VMA is guaranteed to be
+ * stable and not isolated. If the VMA is not found or is being modified the
+ * function returns NULL.
+ */
+struct vm_area_struct *lock_vma_under_rcu(struct mm_struct *mm,
+					  unsigned long address)
+{
+	MA_STATE(mas, &mm->mm_mt, address, address);
+	struct vm_area_struct *vma;
+
+	rcu_read_lock();
+retry:
+	vma = mas_walk(&mas);
+	if (!vma)
+		goto inval;
+
+	/* Only anonymous vmas are supported for now */
+	if (!vma_is_anonymous(vma))
+		goto inval;
+
+	if (!vma_start_read(vma))
+		goto inval;
+
+	/* Check since vm_start/vm_end might change before we lock the VMA */
+	if (unlikely(address < vma->vm_start || address >= vma->vm_end)) {
+		vma_end_read(vma);
+		goto inval;
+	}
+
+	/* Check if the VMA got isolated after we found it */
+	if (vma->detached) {
+		vma_end_read(vma);
+		/* The area was replaced with another one */
+		goto retry;
+	}
+
+	rcu_read_unlock();
+	return vma;
+inval:
+	rcu_read_unlock();
+	return NULL;
+}
+#endif /* CONFIG_PER_VMA_LOCK */
+
 #ifndef __PAGETABLE_P4D_FOLDED
 /*
  * Allocate p4d page table.