From: "Kiryl Shutsemau (Meta)" <[email protected]> Tests scattered through the collapse path decide what a collapse is allowed to do by asking whether khugepaged started it. Between them they settle:
- which VMAs are eligible, and how hard to try for a folio; - how many empty, swapped-out or shared PTEs a window may contain, and whether a sub-PMD window is held to a stricter rule than a PMD; - whether a range has to look used, and whether a MADV_FREE'd page is left alone; - whether the PMD is mapped as part of the request, and whether dirty pages are worth writing back and retrying. None of those is a fact about khugepaged. Each is something the caller decided before asking, and the collapse code should not have to look up who called to find out. Add struct collapse_policy for the caller to fill: khugepaged from its own settings, MADV_COLLAPSE from the fact that a user asked explicitly. Every test becomes a read of a field. khugepaged fills the policy once per scan pass, MADV_COLLAPSE once per call. That is the one change in behaviour: the tunables are sampled once per pass rather than on every call, so a table scanned early in a pass and one scanned late are judged alike. cc->is_khugepaged stays, with a single reader left: the daemon's collapse counter, which is bookkeeping and not policy. collapse_file() also drops a NULL check on the collapse_control. It has one call site, reached only from collapse_single_pmd(), which dereferences cc unconditionally, so the check was already dead. Assisted-by: Claude-Code:claude-opus-5 Signed-off-by: Kiryl Shutsemau (Meta) <[email protected]> --- mm/collapse.h | 47 ++++++++++++++++++++++ mm/khugepaged.c | 105 +++++++++++++++++++++++++++--------------------- 2 files changed, 107 insertions(+), 45 deletions(-) diff --git a/mm/collapse.h b/mm/collapse.h index 9c82e71533df..44f52ea5bbb8 100644 --- a/mm/collapse.h +++ b/mm/collapse.h @@ -2,6 +2,7 @@ #ifndef __MM_COLLAPSE_H #define __MM_COLLAPSE_H +#include <linux/mm.h> #include <linux/nodemask.h> #include <linux/pgtable.h> #include <linux/types.h> @@ -41,7 +42,53 @@ enum scan_result { SCAN_PAGE_DIRTY_OR_WRITEBACK, }; +/* + * What a collapse is allowed to do, decided by whoever asked for it, so the + * code doing it need not ask who its caller is: khugepaged fills this in from + * its own settings, MADV_COLLAPSE from the fact that a user asked explicitly. + */ +struct collapse_policy { + /* Limits, stated per PMD; HPAGE_PMD_NR means "no limit" */ + unsigned int max_ptes_none; + unsigned int max_ptes_swap; + unsigned int max_ptes_shared; + + /* + * Hold a sub-PMD window to a stricter rule than a PMD: no swapped-out + * and no shared PTEs at all, and max_ptes_none as + * collapse_max_ptes_none() scales it. khugepaged holds mTHP collapse + * to that; an explicit request does not. + */ + bool strict_sub_pmd; + + /* + * Collapse only where it looks worth doing: require some sign the range + * is in use, and leave clean lazyfree folios for reclaim rather than + * collapsing them into a folio that is not lazyfree. A user who asked + * for a collapse gets one either way. + */ + bool skip_lazyfree; + bool require_referenced; + + /* + * Finish the job rather than leaving it half done for a fault to pick + * up: map the PMD over a file collapse before returning, and write + * dirty pages back and retry once instead of refusing them. Both cost + * latency the caller has asked to pay. + */ + bool install_pmd; + bool writeback_dirty; + + /* How hard to try for a destination folio */ + gfp_t gfp; + + /* Which VMAs are eligible, as thp_vma_allowable_orders() spells it */ + enum tva_type tva_type; +}; + struct collapse_control { + struct collapse_policy policy; + bool is_khugepaged; /* Num pages scanned per node */ diff --git a/mm/khugepaged.c b/mm/khugepaged.c index a12aafae8d9c..eebc044a930e 100644 --- a/mm/khugepaged.c +++ b/mm/khugepaged.c @@ -310,15 +310,12 @@ struct attribute_group khugepaged_attr_group = { static unsigned int collapse_max_ptes_none(struct collapse_control *cc, struct vm_area_struct *vma, unsigned int order) { - const unsigned int max_ptes_none = khugepaged_max_ptes_none; + const unsigned int max_ptes_none = cc->policy.max_ptes_none; if (vma && userfaultfd_armed(vma)) return 0; - /* for MADV_COLLAPSE, allow any empty/shared zeropage PTEs */ - if (!cc->is_khugepaged) - return HPAGE_PMD_NR; - /* for PMD collapse, respect the user defined maximum */ - if (is_pmd_order(order)) + /* The limit as given, at the PMD order and wherever it is not capped */ + if (is_pmd_order(order) || !cc->policy.strict_sub_pmd) return max_ptes_none; /* * for mTHP collapse with the sysctl value set to KHUGEPAGED_MAX_PTES_LIMIT, @@ -350,19 +347,12 @@ static unsigned int collapse_max_ptes_shared(struct collapse_control *cc, unsigned int order) { /* - * For MADV_COLLAPSE, do not restrict the number of PTEs that map shared - * anonymous pages. + * A sub-PMD window held to the strict rule takes no shared page at all: + * an mTHP is not worth the CoW-breaking. */ - if (!cc->is_khugepaged) - return HPAGE_PMD_NR; - /* - * for mTHP collapse do not allow collapsing anonymous memory pages that - * are shared between processes. - */ - if (!is_pmd_order(order)) + if (!is_pmd_order(order) && cc->policy.strict_sub_pmd) return 0; - /* for PMD collapse, respect the user defined maximum */ - return khugepaged_max_ptes_shared; + return cc->policy.max_ptes_shared; } /** @@ -378,16 +368,12 @@ static unsigned int collapse_max_ptes_swap(struct collapse_control *cc, unsigned int order) { /* - * For MADV_COLLAPSE, do not restrict the number PTEs entries or - * pagecache entries that are non-present. + * A sub-PMD window held to the strict rule takes nothing non-present: + * reading pages back to build an mTHP is not worth the latency. */ - if (!cc->is_khugepaged) - return HPAGE_PMD_NR; - /* for mTHP collapse do not allow any non-present PTEs or pagecache entries */ - if (!is_pmd_order(order)) + if (!is_pmd_order(order) && cc->policy.strict_sub_pmd) return 0; - /* for PMD collapse, respect the user defined maximum */ - return khugepaged_max_ptes_swap; + return cc->policy.max_ptes_swap; } int hugepage_madvise(struct vm_area_struct *vma, @@ -686,7 +672,7 @@ static enum scan_result __collapse_huge_page_isolate(struct vm_area_struct *vma, * If the vma has the VM_DROPPABLE flag, the collapse will * preserve the lazyfree property without needing to skip. */ - if (cc->is_khugepaged && !(vma->vm_flags & VM_DROPPABLE) && + if (cc->policy.skip_lazyfree && !(vma->vm_flags & VM_DROPPABLE) && folio_test_lazyfree(folio) && !pte_dirty(pteval)) { result = SCAN_PAGE_LAZYFREE; goto out; @@ -775,12 +761,12 @@ static enum scan_result __collapse_huge_page_isolate(struct vm_area_struct *vma, if (folio_test_large(folio)) list_add_tail(&folio->lru, compound_pagelist); next: - if (cc->is_khugepaged && + if (cc->policy.require_referenced && folio_pte_referenced(folio, vma, addr, pteval)) referenced++; } - if (unlikely(cc->is_khugepaged && !referenced)) { + if (unlikely(cc->policy.require_referenced && !referenced)) { result = SCAN_LACK_REFERENCED_PAGE; } else { result = SCAN_SUCCEED; @@ -984,6 +970,36 @@ static inline gfp_t alloc_hugepage_khugepaged_gfpmask(void) return khugepaged_defrag() ? GFP_TRANSHUGE : GFP_TRANSHUGE_LIGHT; } +/* khugepaged collapses on its own initiative, so it obeys its own settings. */ +static void collapse_policy_khugepaged(struct collapse_policy *p) +{ + p->max_ptes_none = READ_ONCE(khugepaged_max_ptes_none); + p->max_ptes_swap = READ_ONCE(khugepaged_max_ptes_swap); + p->max_ptes_shared = READ_ONCE(khugepaged_max_ptes_shared); + p->strict_sub_pmd = true; + p->skip_lazyfree = true; + p->require_referenced = true; + p->install_pmd = false; + p->writeback_dirty = false; + p->gfp = alloc_hugepage_khugepaged_gfpmask(); + p->tva_type = TVA_KHUGEPAGED; +} + +/* MADV_COLLAPSE was asked for explicitly, so it is not held to those. */ +static void collapse_policy_forced(struct collapse_policy *p) +{ + p->max_ptes_none = HPAGE_PMD_NR; + p->max_ptes_swap = HPAGE_PMD_NR; + p->max_ptes_shared = HPAGE_PMD_NR; + p->strict_sub_pmd = false; + p->skip_lazyfree = false; + p->require_referenced = false; + p->install_pmd = true; + p->writeback_dirty = true; + p->gfp = GFP_TRANSHUGE; + p->tva_type = TVA_FORCED_COLLAPSE; +} + #ifdef CONFIG_NUMA static int collapse_find_target_node(struct collapse_control *cc) { @@ -1021,8 +1037,7 @@ static enum scan_result hugepage_vma_revalidate(struct mm_struct *mm, unsigned l struct collapse_control *cc, unsigned int order) { struct vm_area_struct *vma; - enum tva_type type = cc->is_khugepaged ? TVA_KHUGEPAGED : - TVA_FORCED_COLLAPSE; + enum tva_type type = cc->policy.tva_type; if (unlikely(collapse_test_exit_or_disable(mm))) return SCAN_ANY_PROCESS; @@ -1205,8 +1220,7 @@ static enum scan_result __collapse_huge_page_swapin(struct mm_struct *mm, static enum scan_result alloc_charge_folio(struct folio **foliop, struct mm_struct *mm, struct collapse_control *cc, unsigned int order) { - gfp_t gfp = (cc->is_khugepaged ? alloc_hugepage_khugepaged_gfpmask() : - GFP_TRANSHUGE); + gfp_t gfp = cc->policy.gfp; int node = collapse_find_target_node(cc); struct folio *folio; @@ -1559,7 +1573,7 @@ static enum scan_result collapse_scan_pmd(struct mm_struct *mm, const unsigned int max_ptes_shared = collapse_max_ptes_shared(cc, HPAGE_PMD_ORDER); const unsigned int max_ptes_swap = collapse_max_ptes_swap(cc, HPAGE_PMD_ORDER); unsigned int max_ptes_none = collapse_max_ptes_none(cc, vma, HPAGE_PMD_ORDER); - enum tva_type tva_flags = cc->is_khugepaged ? TVA_KHUGEPAGED : TVA_FORCED_COLLAPSE; + enum tva_type tva_flags = cc->policy.tva_type; pmd_t *pmd; pte_t *pte, *_pte, pteval; int i; @@ -1658,7 +1672,7 @@ static enum scan_result collapse_scan_pmd(struct mm_struct *mm, * If the vma has the VM_DROPPABLE flag, the collapse will * preserve the lazyfree property without needing to skip. */ - if (cc->is_khugepaged && !(vma->vm_flags & VM_DROPPABLE) && + if (cc->policy.skip_lazyfree && !(vma->vm_flags & VM_DROPPABLE) && folio_test_lazyfree(folio) && !pte_dirty(pteval)) { result = SCAN_PAGE_LAZYFREE; goto out_unmap; @@ -1716,11 +1730,11 @@ static enum scan_result collapse_scan_pmd(struct mm_struct *mm, goto out_unmap; } - if (cc->is_khugepaged && + if (cc->policy.require_referenced && folio_pte_referenced(folio, vma, addr, pteval)) referenced++; } - if (cc->is_khugepaged && + if (cc->policy.require_referenced && (!referenced || (unmapped && referenced < HPAGE_PMD_NR / 2))) { result = SCAN_LACK_REFERENCED_PAGE; @@ -2572,11 +2586,11 @@ static enum scan_result collapse_file(struct mm_struct *mm, unsigned long addr, xas_unlock_irq(&xas); /* - * Remove pte page tables, so we can re-fault the page as huge. - * If MADV_COLLAPSE, adjust result to call try_collapse_pte_mapped_thp(). + * Remove pte page tables, so we can re-fault the page as huge. A caller + * that wants the PMD mapped now is told to go and do that. */ retract_page_tables(mapping, start); - if (cc && !cc->is_khugepaged) + if (cc->policy.install_pmd) result = SCAN_PTE_MAPPED_HUGEPAGE; folio_unlock(new_folio); @@ -2760,11 +2774,8 @@ static enum scan_result collapse_single_pmd(unsigned long addr, retry: result = collapse_scan_file(mm, addr, file, pgoff, cc); - /* - * For MADV_COLLAPSE, when encountering dirty pages, try to writeback, - * then retry the collapse one time. - */ - if (!cc->is_khugepaged && result == SCAN_PAGE_DIRTY_OR_WRITEBACK && + /* Dirty pages are worth a writeback and one more try, if asked for */ + if (cc->policy.writeback_dirty && result == SCAN_PAGE_DIRTY_OR_WRITEBACK && !triggered_wb && mapping_can_writeback(file->f_mapping)) { const loff_t lstart = (loff_t)pgoff << PAGE_SHIFT; const loff_t lend = lstart + HPAGE_PMD_SIZE - 1; @@ -2781,7 +2792,7 @@ static enum scan_result collapse_single_pmd(unsigned long addr, result = SCAN_ANY_PROCESS; else result = try_collapse_pte_mapped_thp(mm, addr, - !cc->is_khugepaged); + cc->policy.install_pmd); if (result == SCAN_PMD_MAPPED) result = SCAN_SUCCEED; mmap_read_unlock(mm); @@ -2931,6 +2942,9 @@ static void khugepaged_do_scan(struct collapse_control *cc) lru_add_drain_all(); + /* One policy for the whole pass, so every table is judged the same */ + collapse_policy_khugepaged(&cc->policy); + cc->progress = 0; while (true) { cond_resched(); @@ -3159,6 +3173,7 @@ int madvise_collapse(struct vm_area_struct *vma, unsigned long start, if (!cc) return -ENOMEM; cc->is_khugepaged = false; + collapse_policy_forced(&cc->policy); cc->progress = 0; mmgrab(mm); -- 2.54.0
