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: 69dbe6daf104 ("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>

1 files changed, 15 insertions, 20 deletions

diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
index 478e2b169c13..0aa5caac5164 100644
--- a/fs/userfaultfd.c
+++ b/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;
 }