Re: [PATCH v2] mm: Fix kernel BUG when userfaultfd_move encounters swapcache

[Barry Song <21cnbao@gmail.com>]

On Wed, Feb 26, 2025 at 2:00 PM Peter Xu <peterx@redhat.com> wrote:
>
> Acked-by: Peter Xu <peterx@redhat.com>
>

Thanks!

> Some nitpicks below, maybe no worth for a repost..

Hi Andrew,

Could you please help squash the below change?

From 42273506ba00723151e3a08b4ffd3f2c303e7ccc Mon Sep 17 00:00:00 2001
From: Barry Song <v-songbaohua@oppo.com>
Date: Wed, 26 Feb 2025 15:22:17 +1300
Subject: [PATCH] minor cleanup according to Peter Xu

According to Peter Xu:
1. Unnecessary line move.
2. Can drop this folio check as it just did check
   "!IS_ERR_OR_NULL(folio)"
3. Not sure if it can do any harm, but maybe still nicer
   to put swap before locking folio.

Signed-off-by: Barry Song <v-songbaohua@oppo.com>
---
 mm/userfaultfd.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
index 2df5d100e76d..2955e20f86bf 100644
--- a/mm/userfaultfd.c
+++ b/mm/userfaultfd.c
@@ -1101,8 +1101,8 @@ static int move_swap_pte(struct mm_struct *mm, struct vm_area_struct *dst_vma,
 
 	orig_src_pte = ptep_get_and_clear(mm, src_addr, src_pte);
 	set_pte_at(mm, dst_addr, dst_pte, orig_src_pte);
-
 	double_pt_unlock(dst_ptl, src_ptl);
+
 	return 0;
 }
 
@@ -1369,7 +1369,7 @@ static int move_pages_pte(struct mm_struct *mm, pmd_t *dst_pmd, pmd_t *src_pmd,
 			folio = filemap_get_folio(swap_address_space(entry),
 					swap_cache_index(entry));
 		if (!IS_ERR_OR_NULL(folio)) {
-			if (folio && folio_test_large(folio)) {
+			if (folio_test_large(folio)) {
 				err = -EBUSY;
 				folio_put(folio);
 				goto out;
@@ -1380,10 +1380,10 @@ static int move_pages_pte(struct mm_struct *mm, pmd_t *dst_pmd, pmd_t *src_pmd,
 				pte_unmap(&orig_src_pte);
 				pte_unmap(&orig_dst_pte);
 				src_pte = dst_pte = NULL;
-				/* now we can block and wait */
-				folio_lock(src_folio);
 				put_swap_device(si);
 				si = NULL;
+				/* now we can block and wait */
+				folio_lock(src_folio);
 				goto retry;
 			}
 		}
-- 
2.39.3 (Apple Git-146)

> Peter Xu
>

Thanks
Barry