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20:21, Lorenzo Stoakes wrote: > > A couple follow up points that occurred to me: > > > > On Sat, Jun 07, 2025 at 12:55:19PM +0100, Lorenzo Stoakes wrote: > > > Not related to your patch at all, but man this whole thing (thp allowed orders) > > > needs significant improvement, it seems always perversely complicated for a > > > relatively simple operation. > > > > > > Overall I LOVE what you're doing here, but I feel we can clarify things a > > > little while we're at it to make it clear exactly what we're doing. > > > > > > This is a very important change so forgive my fiddling about here but I'm > > > hoping we can take the opportunity to make things a little simpler! > > > > > > On Thu, Jun 05, 2025 at 04:00:58PM +0800, Baolin Wang wrote: > > > > 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. > > > > > > > > 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". > > > > > > I'm generally not sure it's worth talking only about MADV_COLLAPSE here when > > > you're changing what THP is permitted across the board, I may have missed some > > > discussion and forgive me if so, but what is special about MADV_COLLAPSE's use > > > of thp_vma_allowable_orders() that makes it ignore 'never's moreso than other > > > users? > > > > I'd mention that MADV_COLLAPSE is special because of not specifying > > TVA_ENFORCE_SYSFS but you are making this change across the board for all > > callers who do not specify this. > > 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 > the 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)) > return SCAN_VMA_CHECK; Thanks for referencing that - it's a nicely worded comment that does explain it indeed :) Definitely worth mentioning that in the commit msg I think. > > > > > I'd also CLEARLY mention that you handle David's request re: madvise by > > restricting yourself to checking only for NEVER and retaining the existing logic > > of not enforcing sysfs settings when TVA_ENFORCE_SYSFS, which includes not > > checking the VMA for VM_HUGEPAGE if the madvise mode is enabled. > > Sure. Thanks! > > > > > (i.e. addressing David's request). > > > > [snip] > > > > > I feel this is compressing a lot of logic in a way that took me several > > > readings to understand (hey I might not be the smartest cookie in the jar, > > > but we need to account for all levels of kernel developer ;) > > > > > > I feel like we can make things a lot clearer here by separating out with a > > > helper function (means we can drop some indentation too), and also take > > > advantage of the fact that, if orders == 0, __thp_vma_allowable_orders() > > > exits with 0 early so no need for us to do so ourselves: > > > > > > /* Strictly mask requested anonymous orders according to sysfs settings. */ > > > static inline unsigned long __thp_mask_anon_orders(unsigned long vm_flags, > > > unsigned long tva_flags, unsigned long orders) > > > { > > > unsigned long always = READ_ONCE(huge_anon_orders_always); > > > unsigned long madvise = READ_ONCE(huge_anon_orders_madvise); > > > unsigned long inherit = READ_ONCE(huge_anon_orders_inherit);; > > > bool inherit_enabled = hugepage_global_enabled(); > > > bool has_madvise = vm_flags & VM_HUGEPAGE; > > > unsigned long mask = always | madvise; > > > > > > mask = always | madvise; > > > if (inherit_enabled) > > > mask |= inherit; > > > > > > /* All set to/inherit NEVER - never means never globally, abort. */ > > > if (!(mask & orders)) > > > return 0; > > > > > > /* Otherwise, we only enforce sysfs settings if asked. */ > > > > Perhaps worth adding a comment here noting that, if the user sets a sysfs mode > > of madvise and if TVA_ENFORCE_SYSFS is not set, we don't bother checking whether > > the VMA has VM_HUGEPAGE set. > > Sure, will do. Thanks for reviewing. Thanks!