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b=JbknlN2pyXxa62LpH3djtmN7GfJwtM9i/YXPq0QOf//aefZu5ftNzsreiJEkq4ILgWZwHY6WeaimaOFh3nzczshO9UBhz0duT5QWAXjFm3tCDqSUtra+5OvO/sTN+j5etKYo0Bdir96yMT127WYtrtNoOAdDEsAmjEbCEQmhVuU= Authentication-Results-Original: dkim=none (message not signed) header.d=none;dmarc=none action=none header.from=arm.com; Received: from AM9PR08MB7120.eurprd08.prod.outlook.com (2603:10a6:20b:3dc::22) by DB9PR08MB6601.eurprd08.prod.outlook.com (2603:10a6:10:23e::18) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.20.8769.21; Thu, 22 May 2025 10:02:31 +0000 Received: from AM9PR08MB7120.eurprd08.prod.outlook.com ([fe80::2933:29aa:2693:d12e]) by AM9PR08MB7120.eurprd08.prod.outlook.com ([fe80::2933:29aa:2693:d12e%2]) with mapi id 15.20.8769.019; Thu, 22 May 2025 10:02:30 +0000 Message-ID: <47ff4f56-6bc0-4e7f-8b71-765a08a4cac4@arm.com> Date: Thu, 22 May 2025 15:32:24 +0530 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] mm/khugepaged: Fix race with folio splitting in hpage_collapse_scan_file() To: Shivank Garg , akpm@linux-foundation.org, david@redhat.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org Cc: ziy@nvidia.com, baolin.wang@linux.alibaba.com, lorenzo.stoakes@oracle.com, Liam.Howlett@oracle.com, npache@redhat.com, ryan.roberts@arm.com, fengwei.yin@intel.com, bharata@amd.com, syzbot+2b99589e33edbe9475ca@syzkaller.appspotmail.com References: <20250522093452.6379-1-shivankg@amd.com> Content-Language: en-US From: Dev Jain In-Reply-To: <20250522093452.6379-1-shivankg@amd.com> Content-Type: text/plain; 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X-OriginatorOrg: arm.com X-MS-Exchange-CrossTenant-OriginalArrivalTime: 22 May 2025 10:03:03.7002 (UTC) X-MS-Exchange-CrossTenant-Network-Message-Id: 145fc17f-69a1-46ed-7e1f-08dd9917d7a9 X-MS-Exchange-CrossTenant-Id: f34e5979-57d9-4aaa-ad4d-b122a662184d X-MS-Exchange-CrossTenant-OriginalAttributedTenantConnectingIp: TenantId=f34e5979-57d9-4aaa-ad4d-b122a662184d;Ip=[4.158.2.129];Helo=[outbound-uk1.az.dlp.m.darktrace.com] X-MS-Exchange-CrossTenant-AuthSource: DU6PEPF0000A7DE.eurprd02.prod.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Anonymous X-MS-Exchange-CrossTenant-FromEntityHeader: HybridOnPrem X-MS-Exchange-Transport-CrossTenantHeadersStamped: GV1PR08MB7801 On 22/05/25 3:04 pm, Shivank Garg wrote: > 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 > Fixes: 05c5323b2a34 ("mm: track mapcount of large folios in single value") > Suggested-by: David Hildenbrand > Signed-off-by: Shivank Garg > --- > 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)) { Do we need to change is_refcount_suitable()? > 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);