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Hildenbrand" , "Matthew Brost" , "Joshua Hahn" , "Rakie Kim" , "Byungchul Park" , "Gregory Price" , "Ying Huang" , "Alistair Popple" , To: "Kefeng Wang" , "Andrew Morton" From: "Zi Yan" Subject: Re: [PATCH] mm: migrate_device: fix pte_pfn/pte_dirty called on non-present PTE X-Mailer: aerc 0.21.0 References: <20260706111958.3649651-1-wangkefeng.wang@huawei.com> <20260706110113.920498961e5b29b212c7a199@linux-foundation.org> <61cebac3-936b-4366-b1fc-35db70a2f1d0@huawei.com> In-Reply-To: <61cebac3-936b-4366-b1fc-35db70a2f1d0@huawei.com> X-ClientProxiedBy: CH0P221CA0034.NAMP221.PROD.OUTLOOK.COM (2603:10b6:610:11d::17) To IA0PR12MB8374.namprd12.prod.outlook.com (2603:10b6:208:40e::7) MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: IA0PR12MB8374:EE_|IA1PR12MB6530:EE_ X-MS-Office365-Filtering-Correlation-Id: 2557cfdb-395a-4bd6-219b-08dedc6a0241 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: 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pte_dirty() have undefined behaviour when called on a >>>> non-present PTE. In migrate_vma_collect_pmd(), these functions may be >>>> invoked on non-present entries (e.g., swap PTEs), leading to potential >>>> crashes from pte_pfn() or incorrect dirty folio accounting from >>>> pte_dirty(). Fix both by guarding with pte_present() checks. >>>> >>>> Fixes: fd35ca3d12cc ("mm/migrate_device.c: copy pte dirty bit to page"= ) >>>> Fixes: a3589e1d5fe3 ("mm/migrate_device.c: add missing flush_cache_pag= e()") >>> >>> Should we cc:stable? > > Yes. > > >>> >>> Sashiko might have found a pre-existing issue: >>> https://sashiko.dev/#/patchset/20260706111958.3649651-1-wangkefeng.wan= g@huawei.com >>=20 >> I looked into the issue and came up with a fix below. It looks ugly, so >> feel free to make a better one out of it. >>=20 >> From 7fa06512638a0febb79a00dba63f12818a85788b Mon Sep 17 00:00:00 2001 >> From: Zi Yan >> Date: Mon, 6 Jul 2026 21:25:29 -0400 >> Subject: [PATCH] mm/migrate_device: avoid scanning/collecting PFNs withi= n a >> PMD >>=20 >> migrate_vma_collect_pmd() can drop pte lock to split a large folio and >> restart. But the code does not handle restart properly and can cause >> collecting inconsistent PFNs and overflowing migrate->dst and migrate->s= rc >> arrays. Fix it by: >> 1. avoiding migrate_vma_collect_huge_pmd() if collection is in progress, >> 2. skipping the rest of the range if pmd no longer points to a pte page. >>=20 >> Fixes: a30b48bf1b244 ("mm/migrate_device: implement THP migration of zon= e device pages") >> Reported-by: Sashiko >> Closes: https://sashiko.dev/#/patchset/20260706111958.3649651-1-wangkefe= ng.wang@huawei.com >> Assisted-by: Claude:claude-opus-4-8 >> Signed-off-by: Zi Yan > > > Thank you for your fix, it looks good, there is some repetition (3 > times) regarding flush+skip in migrate_vma_collect_pmd(), but for bug > fixes, keep it minimal is better. I don't have new ideas to improve it. > OK, let me send it properly. >> --- >> mm/migrate_device.c | 18 ++++++++++++++++-- >> 1 file changed, 16 insertions(+), 2 deletions(-) >>=20 >> diff --git a/mm/migrate_device.c b/mm/migrate_device.c >> index 052167f9ad547..d86b3989d0c59 100644 >> --- a/mm/migrate_device.c >> +++ b/mm/migrate_device.c >> @@ -257,7 +257,11 @@ static int migrate_vma_collect_pmd(pmd_t *pmdp, >> pte_t *ptep; >> =20 >> again: >> - if (pmd_trans_huge(*pmdp) || !pmd_present(*pmdp)) { >> + /* >> + * Only check pmd when addr is at start, namely no pte is scanned or >> + * collected, to avoid collecting inconsistent PFNs. >> + */ >> + if (addr =3D=3D start && (pmd_trans_huge(*pmdp) || !pmd_present(*pmdp)= )) { >> int ret =3D migrate_vma_collect_huge_pmd(pmdp, start, end, walk, fau= lt_folio); >> =20 >> if (ret =3D=3D -EAGAIN) >> @@ -267,8 +271,18 @@ static int migrate_vma_collect_pmd(pmd_t *pmdp, >> } >> =20 >> ptep =3D pte_offset_map_lock(mm, pmdp, start, &ptl); >> - if (!ptep) >> + if (!ptep) { >> + /* >> + * Skip the rest if pmd becomes huge or cleared. Flush if any >> + * pte is modified >> + */ >> + if (addr !=3D start) { >> + if (unmapped) >> + flush_tlb_range(walk->vma, start, end); >> + return migrate_vma_collect_skip(addr, end, walk); >> + } >> goto again; >> + } >> lazy_mmu_mode_enable(); >> ptep +=3D (addr - start) / PAGE_SIZE; >> =20 --=20 Best Regards, Yan, Zi