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riel@surriel.com, liam@infradead.org, vbabka@kernel.org, harry@kernel.org, jannh@google.com, sj@kernel.org, ziy@nvidia.com, lance.yang@linux.dev, linux-mm@kvack.org, linux-kernel@vger.kernel.org, stable@vger.kernel.org Subject: Re: [Patch mm-hotfixes v5] mm/page_vma_mapped: fix device-private PMD handling Message-ID: References: <20260630021540.17297-1-richard.weiyang@gmail.com> Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260630021540.17297-1-richard.weiyang@gmail.com> X-ClientProxiedBy: ME3PR01CA0032.ausprd01.prod.outlook.com (2603:10c6:220:19f::6) To BL0PR12MB4995.namprd12.prod.outlook.com (2603:10b6:208:1c7::23) MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: BL0PR12MB4995:EE_|DS5PPFDB3A23D1A:EE_ X-MS-Office365-Filtering-Correlation-Id: 309840c1-a8fb-49cf-d95c-08ded65bc0d7 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: 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 gu0R5TbK 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 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Tue, Jun 30, 2026 at 02:15:40AM +0000, Wei Yang wrote: > Commit 65edfda6f3f2 ("mm/rmap: extend rmap and migration support > device-private entries") introduced the concept of device-private > PMD entries, but did not correctly update the rmap walk code to > account for them. > > As a result, when page_vma_mapped_walk() encounters device-private > PMD entries, it takes no action other than to acquire the PMD lock > and exit. > > However this is highly problematic for two reasons - firstly, > device private entries possess a PFN so check_pmd() needs to be > called to ensure an overlapping PFN range. > > Secondly, and more importantly, if PVMW_MIGRATION is set the > caller assumes the returned entry is a migration entry, resulting > in memory corruption when the caller tries to interpret the device > private entry as such. > > In addition, commit 146287290023 ("mm/huge_memory: implement > device-private THP splitting") allowed device private PMDs to be > split like THP mappings, but again did not update this code path. > > As a result, we might race a PMD split prior to acquiring the PMD > lock. > > This patch addresses all of these issues by invoking check_pmd(), > ensuring PMVW_MIGRATION is not set and checks whether a split raced > us we do for PMD THP and migration entries. > > Instead of checking for a subset of the cases after taking the > pmd_lock(), put device-private along with pmd_trans_huge() and > pmd_is_migration_entry(). Also remove thp_migration_supported() as > it is already guarded by pmd_is_migration_entry(). > > Fixes: 65edfda6f3f2 ("mm/rmap: extend rmap and migration support device-private entries") > Cc: > Signed-off-by: Wei Yang > Suggested-by: David Hildenbrand > Cc: David Hildenbrand > Cc: Balbir Singh > Cc: SeongJae Park > Cc: Zi Yan > Cc: Lorenzo Stoakes > Cc: Lance Yang > > --- > v5: > * put device-private pmd handling along with the other two cases > * remove thp_migration_supported() > v4: https://lore.kernel.org/all/20260624065353.1622-1-richard.weiyang@gmail.com/T/#u > * refine subject and commit log based on Lorenzo's suggestion > * put pmd device-private entry handling in its own if branch, > suggested by Lorenzo > > v3: > * remove cleanup part, only fix the issue for device-private entry > * refine user effect description based on Lorenzo's suggestion > > v2: https://lore.kernel.org/all/20260616063436.20455-1-richard.weiyang@gmail.com/T/#u > * specify the possible error case of current code and user visible effect > * besides fix, cleanup the pmd entry handling based on David's suggestion > > v1: https://lore.kernel.org/linux-mm/20260508013728.21285-1-richard.weiyang@gmail.com/ > --- > mm/page_vma_mapped.c | 30 ++++++++++++++++-------------- > 1 file changed, 16 insertions(+), 14 deletions(-) > > diff --git a/mm/page_vma_mapped.c b/mm/page_vma_mapped.c > index 2ccbabfb2cc1..2d6c58488e3a 100644 > --- a/mm/page_vma_mapped.c > +++ b/mm/page_vma_mapped.c > @@ -243,21 +243,30 @@ bool page_vma_mapped_walk(struct page_vma_mapped_walk *pvmw) > */ > pmde = pmdp_get_lockless(pvmw->pmd); > > - if (pmd_trans_huge(pmde) || pmd_is_migration_entry(pmde)) { > + if (pmd_trans_huge(pmde) || pmd_is_migration_entry(pmde) || > + pmd_is_device_private_entry(pmde)) { > pvmw->ptl = pmd_lock(mm, pvmw->pmd); > pmde = *pvmw->pmd; > - if (!pmd_present(pmde)) { > + if (pmd_is_migration_entry(pmde)) { > softleaf_t entry; > > - if (!thp_migration_supported() || > - !(pvmw->flags & PVMW_MIGRATION)) > + if (!(pvmw->flags & PVMW_MIGRATION)) > return not_found(pvmw); > entry = softleaf_from_pmd(pmde); > + if (!check_pmd(softleaf_to_pfn(entry), pvmw)) > + return not_found(pvmw); > + return true; > + } else if (pmd_is_device_private_entry(pmde)) { > + softleaf_t entry; > > - if (!softleaf_is_migration(entry) || > - !check_pmd(softleaf_to_pfn(entry), pvmw)) > + if (pvmw->flags & PVMW_MIGRATION) > + return not_found(pvmw); > + entry = softleaf_from_pmd(pmde); > + if (!check_pmd(softleaf_to_pfn(entry), pvmw)) > return not_found(pvmw); > return true; > + } else if (!pmd_present(pmde)) { > + return not_found(pvmw); > } > if (likely(pmd_trans_huge(pmde))) { > if (pvmw->flags & PVMW_MIGRATION) > @@ -266,17 +275,10 @@ bool page_vma_mapped_walk(struct page_vma_mapped_walk *pvmw) > return not_found(pvmw); > return true; > } > - /* THP pmd was split under us: handle on pte level */ > + /* THP/device-private pmd was split under us: handle on pte level */ > spin_unlock(pvmw->ptl); > pvmw->ptl = NULL; > } else if (!pmd_present(pmde)) { > - const softleaf_t entry = softleaf_from_pmd(pmde); > - > - if (softleaf_is_device_private(entry)) { > - pvmw->ptl = pmd_lock(mm, pvmw->pmd); > - return true; > - } > - > if ((pvmw->flags & PVMW_SYNC) && > thp_vma_suitable_order(vma, pvmw->address, > PMD_ORDER) && > -- Thanks! Acked-by: Balbir Singh