userfaultfd: fix regression in userfaultfd_unmap_prep()

Android reported a performance regression in the userfaultfd unmap path. 
A closer inspection on the userfaultfd_unmap_prep() change showed that a
second tree walk would be necessary in the reworked code.

Fix the regression by passing each VMA that will be unmapped through to
the userfaultfd_unmap_prep() function as they are added to the unmap list,
instead of re-walking the tree for the VMA.

Link: https://lkml.kernel.org/r/20230601015402.2819343-1-Liam.Howlett@oracle.com
Fixes: 69dbe6da ("userfaultfd: use maple tree iterator to iterate VMAs")
Signed-off-by: Liam R. Howlett <Liam.Howlett@oracle.com>
Reported-by: Suren Baghdasaryan <surenb@google.com>
Suggested-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

fs/userfaultfd.c

@@ -852,31 +852,26 @@ static bool has_unmap_ctx(struct userfaultfd_ctx *ctx, struct list_head *unmaps,
 	return false;
 }
 
-int userfaultfd_unmap_prep(struct mm_struct *mm, unsigned long start,
+int userfaultfd_unmap_prep(struct vm_area_struct *vma, unsigned long start,
 			   unsigned long end, struct list_head *unmaps)
 {
-	VMA_ITERATOR(vmi, mm, start);
-	struct vm_area_struct *vma;
-
-	for_each_vma_range(vmi, vma, end) {
-		struct userfaultfd_unmap_ctx *unmap_ctx;
-		struct userfaultfd_ctx *ctx = vma->vm_userfaultfd_ctx.ctx;
+	struct userfaultfd_unmap_ctx *unmap_ctx;
+	struct userfaultfd_ctx *ctx = vma->vm_userfaultfd_ctx.ctx;
 
-		if (!ctx || !(ctx->features & UFFD_FEATURE_EVENT_UNMAP) ||
-		    has_unmap_ctx(ctx, unmaps, start, end))
-			continue;
+	if (!ctx || !(ctx->features & UFFD_FEATURE_EVENT_UNMAP) ||
+	    has_unmap_ctx(ctx, unmaps, start, end))
+		return 0;
 
-		unmap_ctx = kzalloc(sizeof(*unmap_ctx), GFP_KERNEL);
-		if (!unmap_ctx)
-			return -ENOMEM;
+	unmap_ctx = kzalloc(sizeof(*unmap_ctx), GFP_KERNEL);
+	if (!unmap_ctx)
+		return -ENOMEM;
 
-		userfaultfd_ctx_get(ctx);
-		atomic_inc(&ctx->mmap_changing);
-		unmap_ctx->ctx = ctx;
-		unmap_ctx->start = start;
-		unmap_ctx->end = end;
-		list_add_tail(&unmap_ctx->list, unmaps);
-	}
+	userfaultfd_ctx_get(ctx);
+	atomic_inc(&ctx->mmap_changing);
+	unmap_ctx->ctx = ctx;
+	unmap_ctx->start = start;
+	unmap_ctx->end = end;
+	list_add_tail(&unmap_ctx->list, unmaps);
 
 	return 0;
 }

include/linux/userfaultfd_k.h

@@ -188,8 +188,8 @@ extern bool userfaultfd_remove(struct vm_area_struct *vma,
 			       unsigned long start,
 			       unsigned long end);
 
-extern int userfaultfd_unmap_prep(struct mm_struct *mm, unsigned long start,
-				  unsigned long end, struct list_head *uf);
+extern int userfaultfd_unmap_prep(struct vm_area_struct *vma,
+		unsigned long start, unsigned long end, struct list_head *uf);
 extern void userfaultfd_unmap_complete(struct mm_struct *mm,
 				       struct list_head *uf);
 extern bool userfaultfd_wp_unpopulated(struct vm_area_struct *vma);
@@ -271,7 +271,7 @@ static inline bool userfaultfd_remove(struct vm_area_struct *vma,
 	return true;
 }
 
-static inline int userfaultfd_unmap_prep(struct mm_struct *mm,
+static inline int userfaultfd_unmap_prep(struct vm_area_struct *vma,
 					 unsigned long start, unsigned long end,
 					 struct list_head *uf)
 {

mm/mmap.c

@@ -2417,6 +2417,21 @@ do_vmi_align_munmap(struct vma_iterator *vmi, struct vm_area_struct *vma,
 			goto munmap_sidetree_failed;
 
 		count++;
+		if (unlikely(uf)) {
+			/*
+			 * If userfaultfd_unmap_prep returns an error the vmas
+			 * will remain split, but userland will get a
+			 * highly unexpected error anyway. This is no
+			 * different than the case where the first of the two
+			 * __split_vma fails, but we don't undo the first
+			 * split, despite we could. This is unlikely enough
+			 * failure that it's not worth optimizing it for.
+			 */
+			error = userfaultfd_unmap_prep(next, start, end, uf);
+
+			if (error)
+				goto userfaultfd_error;
+		}
 #ifdef CONFIG_DEBUG_VM_MAPLE_TREE
 		BUG_ON(next->vm_start < start);
 		BUG_ON(next->vm_start > end);
@@ -2429,22 +2444,6 @@ do_vmi_align_munmap(struct vma_iterator *vmi, struct vm_area_struct *vma,
 	if (!next)
 		next = vma_next(vmi);
 
-	if (unlikely(uf)) {
-		/*
-		 * If userfaultfd_unmap_prep returns an error the vmas
-		 * will remain split, but userland will get a
-		 * highly unexpected error anyway. This is no
-		 * different than the case where the first of the two
-		 * __split_vma fails, but we don't undo the first
-		 * split, despite we could. This is unlikely enough
-		 * failure that it's not worth optimizing it for.
-		 */
-		error = userfaultfd_unmap_prep(mm, start, end, uf);
-
-		if (error)
-			goto userfaultfd_error;
-	}
-
 #if defined(CONFIG_DEBUG_VM_MAPLE_TREE)
 	/* Make sure no VMAs are about to be lost. */
 	{