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Message-ID: <8524c7c7-024f-4f17-9b89-ef9aedfca672@arm.com>
Date: Wed, 19 Feb 2025 15:39:15 +0000
From: Ryan Roberts <ryan.roberts@....com>
To: Nico Pache <npache@...hat.com>, linux-kernel@...r.kernel.org,
linux-trace-kernel@...r.kernel.org, linux-mm@...ck.org
Cc: anshuman.khandual@....com, catalin.marinas@....com, cl@...two.org,
vbabka@...e.cz, mhocko@...e.com, apopple@...dia.com,
dave.hansen@...ux.intel.com, will@...nel.org, baohua@...nel.org,
jack@...e.cz, srivatsa@...il.mit.edu, haowenchao22@...il.com,
hughd@...gle.com, aneesh.kumar@...nel.org, yang@...amperecomputing.com,
peterx@...hat.com, ioworker0@...il.com, wangkefeng.wang@...wei.com,
ziy@...dia.com, jglisse@...gle.com, surenb@...gle.com,
vishal.moola@...il.com, zokeefe@...gle.com, zhengqi.arch@...edance.com,
jhubbard@...dia.com, 21cnbao@...il.com, willy@...radead.org,
kirill.shutemov@...ux.intel.com, david@...hat.com, aarcange@...hat.com,
raquini@...hat.com, dev.jain@....com, sunnanyong@...wei.com,
usamaarif642@...il.com, audra@...hat.com, akpm@...ux-foundation.org,
rostedt@...dmis.org, mathieu.desnoyers@...icios.com, tiwai@...e.de
Subject: Re: [RFC v2 5/9] khugepaged: generalize __collapse_huge_page_* for
mTHP support
On 11/02/2025 00:30, Nico Pache wrote:
> generalize the order of the __collapse_huge_page_* functions
> to support future mTHP collapse.
>
> mTHP collapse can suffer from incosistant behavior, and memory waste
> "creep". disable swapin and shared support for mTHP collapse.
>
> No functional changes in this patch.
>
> Signed-off-by: Nico Pache <npache@...hat.com>
> ---
> mm/khugepaged.c | 48 ++++++++++++++++++++++++++++--------------------
> 1 file changed, 28 insertions(+), 20 deletions(-)
>
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index 0cfcdc11cabd..3776055bd477 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -565,15 +565,17 @@ static int __collapse_huge_page_isolate(struct vm_area_struct *vma,
> unsigned long address,
> pte_t *pte,
> struct collapse_control *cc,
> - struct list_head *compound_pagelist)
> + struct list_head *compound_pagelist,
> + u8 order)
nit: I think we are mostly standardised on order being int. Is there any reason
to make it u8 here?
> {
> struct page *page = NULL;
> struct folio *folio = NULL;
> pte_t *_pte;
> int none_or_zero = 0, shared = 0, result = SCAN_FAIL, referenced = 0;
> bool writable = false;
> + int scaled_none = khugepaged_max_ptes_none >> (HPAGE_PMD_ORDER - order);
>
> - for (_pte = pte; _pte < pte + HPAGE_PMD_NR;
> + for (_pte = pte; _pte < pte + (1 << order);
> _pte++, address += PAGE_SIZE) {
> pte_t pteval = ptep_get(_pte);
> if (pte_none(pteval) || (pte_present(pteval) &&
> @@ -581,7 +583,7 @@ static int __collapse_huge_page_isolate(struct vm_area_struct *vma,
> ++none_or_zero;
> if (!userfaultfd_armed(vma) &&
> (!cc->is_khugepaged ||
> - none_or_zero <= khugepaged_max_ptes_none)) {
> + none_or_zero <= scaled_none)) {
> continue;
> } else {
> result = SCAN_EXCEED_NONE_PTE;
> @@ -609,8 +611,8 @@ static int __collapse_huge_page_isolate(struct vm_area_struct *vma,
> /* See khugepaged_scan_pmd(). */
> if (folio_likely_mapped_shared(folio)) {
> ++shared;
> - if (cc->is_khugepaged &&
> - shared > khugepaged_max_ptes_shared) {
> + if (order != HPAGE_PMD_ORDER || (cc->is_khugepaged &&
> + shared > khugepaged_max_ptes_shared)) {
> result = SCAN_EXCEED_SHARED_PTE;
> count_vm_event(THP_SCAN_EXCEED_SHARED_PTE);
Same comment about events; I think you will want to be careful to only count
events for PMD-sized THP using count_vm_event() and introduce equivalent MTHP
events to cover all sizes.
> goto out;
> @@ -711,14 +713,15 @@ static void __collapse_huge_page_copy_succeeded(pte_t *pte,
> struct vm_area_struct *vma,
> unsigned long address,
> spinlock_t *ptl,
> - struct list_head *compound_pagelist)
> + struct list_head *compound_pagelist,
> + u8 order)
> {
> struct folio *src, *tmp;
> pte_t *_pte;
> pte_t pteval;
>
> - for (_pte = pte; _pte < pte + HPAGE_PMD_NR;
> - _pte++, address += PAGE_SIZE) {
> + for (_pte = pte; _pte < pte + (1 << order);
> + _pte++, address += PAGE_SIZE) {
nit: you changed the indentation here.
> pteval = ptep_get(_pte);
> if (pte_none(pteval) || is_zero_pfn(pte_pfn(pteval))) {
> add_mm_counter(vma->vm_mm, MM_ANONPAGES, 1);
> @@ -764,7 +767,8 @@ static void __collapse_huge_page_copy_failed(pte_t *pte,
> pmd_t *pmd,
> pmd_t orig_pmd,
> struct vm_area_struct *vma,
> - struct list_head *compound_pagelist)
> + struct list_head *compound_pagelist,
> + u8 order)
> {
> spinlock_t *pmd_ptl;
>
> @@ -781,7 +785,7 @@ static void __collapse_huge_page_copy_failed(pte_t *pte,
> * Release both raw and compound pages isolated
> * in __collapse_huge_page_isolate.
> */
> - release_pte_pages(pte, pte + HPAGE_PMD_NR, compound_pagelist);
> + release_pte_pages(pte, pte + (1 << order), compound_pagelist);
> }
>
> /*
> @@ -802,7 +806,7 @@ static void __collapse_huge_page_copy_failed(pte_t *pte,
> static int __collapse_huge_page_copy(pte_t *pte, struct folio *folio,
> pmd_t *pmd, pmd_t orig_pmd, struct vm_area_struct *vma,
> unsigned long address, spinlock_t *ptl,
> - struct list_head *compound_pagelist)
> + struct list_head *compound_pagelist, u8 order)
> {
> unsigned int i;
> int result = SCAN_SUCCEED;
> @@ -810,7 +814,7 @@ static int __collapse_huge_page_copy(pte_t *pte, struct folio *folio,
> /*
> * Copying pages' contents is subject to memory poison at any iteration.
> */
> - for (i = 0; i < HPAGE_PMD_NR; i++) {
> + for (i = 0; i < (1 << order); i++) {
> pte_t pteval = ptep_get(pte + i);
> struct page *page = folio_page(folio, i);
> unsigned long src_addr = address + i * PAGE_SIZE;
> @@ -829,10 +833,10 @@ static int __collapse_huge_page_copy(pte_t *pte, struct folio *folio,
>
> if (likely(result == SCAN_SUCCEED))
> __collapse_huge_page_copy_succeeded(pte, vma, address, ptl,
> - compound_pagelist);
> + compound_pagelist, order);
> else
> __collapse_huge_page_copy_failed(pte, pmd, orig_pmd, vma,
> - compound_pagelist);
> + compound_pagelist, order);
>
> return result;
> }
> @@ -1000,11 +1004,11 @@ static int check_pmd_still_valid(struct mm_struct *mm,
> static int __collapse_huge_page_swapin(struct mm_struct *mm,
> struct vm_area_struct *vma,
> unsigned long haddr, pmd_t *pmd,
> - int referenced)
> + int referenced, u8 order)
> {
> int swapped_in = 0;
> vm_fault_t ret = 0;
> - unsigned long address, end = haddr + (HPAGE_PMD_NR * PAGE_SIZE);
> + unsigned long address, end = haddr + (PAGE_SIZE << order);
> int result;
> pte_t *pte = NULL;
> spinlock_t *ptl;
> @@ -1035,6 +1039,11 @@ static int __collapse_huge_page_swapin(struct mm_struct *mm,
> if (!is_swap_pte(vmf.orig_pte))
> continue;
>
> + if (order != HPAGE_PMD_ORDER) {
> + result = SCAN_EXCEED_SWAP_PTE;
> + goto out;
> + }
A comment to explain the rationale for this divergent behaviour based on order
would be helpful.
> +
> vmf.pte = pte;
> vmf.ptl = ptl;
> ret = do_swap_page(&vmf);
> @@ -1114,7 +1123,6 @@ static int collapse_huge_page(struct mm_struct *mm, unsigned long address,
> int result = SCAN_FAIL;
> struct vm_area_struct *vma;
> struct mmu_notifier_range range;
> -
nit: no need for this whitespace change?
> VM_BUG_ON(address & ~HPAGE_PMD_MASK);
>
> /*
> @@ -1149,7 +1157,7 @@ static int collapse_huge_page(struct mm_struct *mm, unsigned long address,
> * that case. Continuing to collapse causes inconsistency.
> */
> result = __collapse_huge_page_swapin(mm, vma, address, pmd,
> - referenced);
> + referenced, HPAGE_PMD_ORDER);
> if (result != SCAN_SUCCEED)
> goto out_nolock;
> }
> @@ -1196,7 +1204,7 @@ static int collapse_huge_page(struct mm_struct *mm, unsigned long address,
> pte = pte_offset_map_lock(mm, &_pmd, address, &pte_ptl);
> if (pte) {
> result = __collapse_huge_page_isolate(vma, address, pte, cc,
> - &compound_pagelist);
> + &compound_pagelist, HPAGE_PMD_ORDER);
> spin_unlock(pte_ptl);
> } else {
> result = SCAN_PMD_NULL;
> @@ -1226,7 +1234,7 @@ static int collapse_huge_page(struct mm_struct *mm, unsigned long address,
>
> result = __collapse_huge_page_copy(pte, folio, pmd, _pmd,
> vma, address, pte_ptl,
> - &compound_pagelist);
> + &compound_pagelist, HPAGE_PMD_ORDER);
> pte_unmap(pte);
> if (unlikely(result != SCAN_SUCCEED))
> goto out_up_write;
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