From: Suren Baghdasaryan <surenb@google.com>
Date: Mon, 27 Feb 2023 17:36:26 +0000 (-0800)
Subject: mm: prevent userfaults to be handled under per-vma lock
X-Git-Url: http://git.maquefel.me/?a=commitdiff_plain;h=444eeb17437a0ef526c606e9141a415d3b7dfddd;p=linux.git

mm: prevent userfaults to be handled under per-vma lock

Due to the possibility of handle_userfault dropping mmap_lock, avoid fault
handling under VMA lock and retry holding mmap_lock.  This can be handled
more gracefully in the future.

Link: https://lkml.kernel.org/r/20230227173632.3292573-28-surenb@google.com
Signed-off-by: Suren Baghdasaryan <surenb@google.com>
Suggested-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

diff --git a/mm/memory.c b/mm/memory.c
index 70f4b1222e5d0..55ac9cdfd3983 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -5266,6 +5266,15 @@ retry:
 	if (!vma_start_read(vma))
 		goto inval;
 
+	/*
+	 * Due to the possibility of userfault handler dropping mmap_lock, avoid
+	 * it for now and fall back to page fault handling under mmap_lock.
+	 */
+	if (userfaultfd_armed(vma)) {
+		vma_end_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);