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X-OriginatorOrg: arm.com X-MS-Exchange-CrossTenant-OriginalArrivalTime: 25 Jun 2025 04:34:54.6711 (UTC) X-MS-Exchange-CrossTenant-Network-Message-Id: 9916b6e2-cffa-4df4-708d-08ddb3a1a222 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: DB1PEPF000509ED.eurprd03.prod.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Anonymous X-MS-Exchange-CrossTenant-FromEntityHeader: HybridOnPrem X-MS-Exchange-Transport-CrossTenantHeadersStamped: PR3PR08MB5593 On 25/06/25 7:10 am, 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)) > " > > Another rule for madvise, referring to David's suggestion: “allowing for > collapsing in a VM without VM_HUGEPAGE in the "madvise" mode would be fine". > > To address this issue, the current strategy should be: > > If no hugepage modes are enabled for the desired orders, nor can we enable them > by inheriting from a 'global' enabled setting - then it must be the case that > all desired orders either specify or inherit 'NEVER' - and we must abort. > > Meanwhile, we should fix the khugepaged selftest for MADV_COLLAPSE. Originally, > we could prevent khugepaged by setting THP_MADVISE and removing MADV_HUGEPAGE > setting, while madvise_collapse() can still perform THP collapse. However, > this would cause some test cases to fail because some tests previously set > MADV_NOHUGEPAGE, and now there is no other way to clear the MADV_NOHUGEPAGE > flag except for setting MADV_HUGEPAGE. Therefore, it should be changed to > THP_ALWAYS here to allow madvise_collapse() to perform THP collapse. > > Suggested-by: David Hildenbrand > Suggested-by: Lorenzo Stoakes > Reviewed-by: Zi Yan > Reviewed-by: Lorenzo Stoakes > Signed-off-by: Baolin Wang > --- > Reviewed-by: Dev Jain >