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SubjectRe: [syzbot] [mm?] WARNING in folio_large_mapcount
From
From e77693b67a8c032d636f6c0bc3f179c4e9cc1133 Mon Sep 17 00:00:00 2001
From: Shivank Garg <shivankg@amd.com>
Date: Mon, 19 May 2025 20:19:32 +0000
Subject: [PATCH] mm/khugepaged: Fix race with folio splitting in
hpage_collapse_scan_file()

folio_mapcount() checks folio_test_large() before proceeding to
folio_large_mapcount(), but there exists a race window where a folio
could be split between these checks which triggered the
VM_WARN_ON_FOLIO(!folio_test_large(folio), folio) in
folio_large_mapcount().

Take a temporary folio reference in hpage_collapse_scan_file() to prevent
races with concurrent folio splitting/freeing. This prevent potential
incorrect large folio detection.

Reported-by: syzbot+2b99589e33edbe9475ca@syzkaller.appspotmail.com
Closes: https://lore.kernel.org/all/6828470d.a70a0220.38f255.000c.GAE@google.com
Suggested-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Shivank Garg <shivankg@amd.com>
---
mm/khugepaged.c | 16 ++++++++++++++++
1 file changed, 16 insertions(+)

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index cc945c6ab3bd..6e8902f9d88c 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -2295,6 +2295,17 @@ static int hpage_collapse_scan_file(struct mm_struct *mm, unsigned long addr,
continue;
}

+ if (!folio_try_get(folio)) {
+ xas_reset(&xas);
+ continue;
+ }
+
+ if (unlikely(folio != xas_reload(&xas))) {
+ folio_put(folio);
+ xas_reset(&xas);
+ continue;
+ }
+
if (folio_order(folio) == HPAGE_PMD_ORDER &&
folio->index == start) {
/* Maybe PMD-mapped */
@@ -2305,23 +2316,27 @@ static int hpage_collapse_scan_file(struct mm_struct *mm, unsigned long addr,
* it's safe to skip LRU and refcount checks before
* returning.
*/
+ folio_put(folio);
break;
}

node = folio_nid(folio);
if (hpage_collapse_scan_abort(node, cc)) {
result = SCAN_SCAN_ABORT;
+ folio_put(folio);
break;
}
cc->node_load[node]++;

if (!folio_test_lru(folio)) {
result = SCAN_PAGE_LRU;
+ folio_put(folio);
break;
}

if (!is_refcount_suitable(folio)) {
result = SCAN_PAGE_COUNT;
+ folio_put(folio);
break;
}

@@ -2333,6 +2348,7 @@ static int hpage_collapse_scan_file(struct mm_struct *mm, unsigned long addr,
*/

present += folio_nr_pages(folio);
+ folio_put(folio);

if (need_resched()) {
xas_pause(&xas);
--
2.34.1
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