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Howlett" , Nico Pache , Ryan Roberts , Dev Jain , Barry Song , Lyude Paul , Danilo Krummrich , David Airlie , Simona Vetter , Ralph Campbell , =?UTF-8?Q?Mika_Penttil=C3=A4?= , Matthew Brost , Francois Dugast References: <20250916122128.2098535-1-balbirs@nvidia.com> <20250916122128.2098535-6-balbirs@nvidia.com> <3FE3F2B9-86F9-4EED-863A-093FE3959B1A@nvidia.com> <62fb7aca-cd30-4058-bfd1-d08f78d5c38d@nvidia.com> <20250923155640.lbtvyi54ueyv2tjb@wrangler> Content-Language: en-US From: Balbir Singh In-Reply-To: <20250923155640.lbtvyi54ueyv2tjb@wrangler> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-ClientProxiedBy: BYAPR02CA0035.namprd02.prod.outlook.com (2603:10b6:a02:ee::48) To PH8PR12MB7277.namprd12.prod.outlook.com (2603:10b6:510:223::13) Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: PH8PR12MB7277:EE_|MW4PR12MB7237:EE_ X-MS-Office365-Filtering-Correlation-Id: bbbb9f92-a019-4aa4-a73d-08ddfb256e0b X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|366016|376014|7416014|1800799024|10070799003; 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Howlett" >>>> Cc: Nico Pache >>>> Cc: Ryan Roberts >>>> Cc: Dev Jain >>>> Cc: Barry Song >>>> Cc: Lyude Paul >>>> Cc: Danilo Krummrich >>>> Cc: David Airlie >>>> Cc: Simona Vetter >>>> Cc: Ralph Campbell >>>> Cc: Mika Penttilä >>>> Cc: Matthew Brost >>>> Cc: Francois Dugast >>>> --- >>>> mm/migrate_device.c | 82 +++++++++++++++++++++++++++++++++++++++++++++ >>>> 1 file changed, 82 insertions(+) >>>> >>>> diff --git a/mm/migrate_device.c b/mm/migrate_device.c >>>> index abd9f6850db6..70c0601f70ea 100644 >>>> --- a/mm/migrate_device.c >>>> +++ b/mm/migrate_device.c >>>> @@ -54,6 +54,53 @@ static int migrate_vma_collect_hole(unsigned long start, >>>> return 0; >>>> } >>>> >>>> +/** >>>> + * migrate_vma_split_folio() - Helper function to split a THP folio >>>> + * @folio: the folio to split >>>> + * @fault_page: struct page associated with the fault if any >>>> + * >>>> + * Returns 0 on success >>>> + */ >>>> +static int migrate_vma_split_folio(struct folio *folio, >>>> + struct page *fault_page) >>>> +{ >>>> + int ret; >>>> + struct folio *fault_folio = fault_page ? page_folio(fault_page) : NULL; >>>> + struct folio *new_fault_folio = NULL; >>>> + >>>> + if (folio != fault_folio) { >>>> + folio_get(folio); >>>> + folio_lock(folio); >>>> + } >>>> + >>>> + ret = split_folio(folio); >>>> + if (ret) { >>>> + if (folio != fault_folio) { >>>> + folio_unlock(folio); >>>> + folio_put(folio); >>>> + } >>>> + return ret; >>>> + } >>>> + >>>> + new_fault_folio = fault_page ? page_folio(fault_page) : NULL; >>>> + >>>> + /* >>>> + * Ensure the lock is held on the correct >>>> + * folio after the split >>>> + */ >>>> + if (!new_fault_folio) { >>>> + folio_unlock(folio); >>>> + folio_put(folio); >>>> + } else if (folio != new_fault_folio) { >>>> + folio_get(new_fault_folio); >>>> + folio_lock(new_fault_folio); >>>> + folio_unlock(folio); >>>> + folio_put(folio); >>>> + } >>>> + >>>> + return 0; >>>> +} >>>> + >>>> static int migrate_vma_collect_pmd(pmd_t *pmdp, >>>> unsigned long start, >>>> unsigned long end, >>>> @@ -136,6 +183,8 @@ static int migrate_vma_collect_pmd(pmd_t *pmdp, >>>> * page table entry. Other special swap entries are not >>>> * migratable, and we ignore regular swapped page. >>>> */ >>>> + struct folio *folio; >>>> + >>>> entry = pte_to_swp_entry(pte); >>>> if (!is_device_private_entry(entry)) >>>> goto next; >>>> @@ -147,6 +196,23 @@ static int migrate_vma_collect_pmd(pmd_t *pmdp, >>>> pgmap->owner != migrate->pgmap_owner) >>>> goto next; >>>> >>>> + folio = page_folio(page); >>>> + if (folio_test_large(folio)) { >>>> + int ret; >>>> + >>>> + pte_unmap_unlock(ptep, ptl); >>>> + ret = migrate_vma_split_folio(folio, >>>> + migrate->fault_page); >>>> + >>>> + if (ret) { >>>> + ptep = pte_offset_map_lock(mm, pmdp, addr, &ptl); >>>> + goto next; >>>> + } >>>> + >>>> + addr = start; >>>> + goto again; >>>> + } >>> >>> This does not look right to me. >>> >>> The folio here is device private, but migrate_vma_split_folio() >>> calls split_folio(), which cannot handle device private folios yet. >>> Your change to split_folio() is in Patch 10 and should be moved >>> before this patch. >>> >> >> Patch 10 is to split the folio in the middle of migration (when we have >> converted the entries to migration entries). This patch relies on the >> changes in patch 4. I agree the names are confusing, I'll reword the >> functions > > Hi Balbir, > > I am still reviewing the patches, but I think I agree with Zi here. > > split_folio() will replace the PMD mappings of the huge folio with PTE > mappings, but will also split the folio into smaller folios. The former > is ok with this patch, but the latter is probably not correct if the folio > is a zone device folio. The driver needs to know about the change, as > usually the driver will have some sort of mapping between GPU physical > memory chunks and their corresponding zone device pages. > Yes, at this point there is no support for split folio callback. I can move this bit to a later patch or move the entire patch to after patch 10. I suspect this is a theoretical bisection concern for a future driver using large folios? Thanks, Balbir