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X-OriginatorOrg: arm.com X-MS-Exchange-CrossTenant-OriginalArrivalTime: 24 Jun 2025 14:43:01.1910 (UTC) X-MS-Exchange-CrossTenant-Network-Message-Id: 1816c587-6953-4ac0-f573-08ddb32d6b60 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: DU6PEPF0000B620.eurprd02.prod.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Anonymous X-MS-Exchange-CrossTenant-FromEntityHeader: HybridOnPrem X-MS-Exchange-Transport-CrossTenantHeadersStamped: AM8PR08MB6338 On 24/06/25 7:38 pm, Baolin Wang wrote: > > > On 2025/6/24 17:57, Baolin Wang wrote: >> >> >> On 2025/6/24 16:41, Dev Jain wrote: >>> >>> On 23/06/25 1:58 pm, Baolin Wang wrote: >>>> When invoking thp_vma_allowable_orders(), the TVA_ENFORCE_SYSFS >>>> flag is not >>>> specified, we will ignore the THP sysfs settings. Whilst it makes >>>> sense for the >>>> callers who do not specify this flag, it creates a odd and >>>> surprising situation >>>> where a sysadmin specifying 'never' for all THP sizes still >>>> observing THP pages >>>> being allocated and used on the system. >>>> >>>> The motivating case for this is MADV_COLLAPSE. The MADV_COLLAPSE >>>> will ignore >>>> the system-wide Anon THP sysfs settings, which means that even >>>> though we have >>>> disabled the Anon THP configuration, MADV_COLLAPSE will still >>>> attempt to collapse >>>> into a Anon THP. This violates the rule we have agreed upon: never >>>> means never. >>>> >>>> Currently, besides MADV_COLLAPSE not setting TVA_ENFORCE_SYSFS, >>>> there is only >>>> one other instance where TVA_ENFORCE_SYSFS is not set, which is in the >>>> collapse_pte_mapped_thp() function, but I believe this is >>>> reasonable from its >>>> comments: >>>> >>>> " >>>> /* >>>>   * If we are here, we've succeeded in replacing all the native pages >>>>   * in the page cache with a single hugepage. If a mm were to fault-in >>>>   * this memory (mapped by a suitably aligned VMA), we'd get the >>>> hugepage >>>>   * and map it by a PMD, regardless of sysfs THP settings. As such, >>>> let's >>>>   * analogously elide sysfs THP settings here. >>>>   */ >>>> if (!thp_vma_allowable_order(vma, vma->vm_flags, 0, PMD_ORDER)) >>> >>> So the behaviour now is: First check whether THP settings converge >>> to never. >>> Then, if enforce_sysfs is not set, return immediately. So in this >>> khugepaged >>> code will it be better to call __thp_vma_allowable_orders()? If the >>> sysfs >>> settings are changed to never before hitting collapse_pte_mapped_thp(), >>> then right now we will return SCAN_VMA_CHECK from here, whereas, the >>> comment >>> says "regardless of sysfs THP settings", which should include >>> "regardless >>> of whether the sysfs settings say never". >> >> Sounds reasonable to me. Thanks. >> >> I will change thp_vma_allowable_order() to >> __thp_vma_allowable_orders() in the collapse_pte_mapped_thp() >> function to maintain consistency with the original logic. >> >> Lorenzo and David, how do you think? Thanks. > > After thinking more, since collapse_pte_mapped_thp() is only used for > file/shmem collapse, changing to __thp_vma_allowable_orders() has no > effect. So I prefer to leave it as is. Oops my bad, thanks.