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Message-ID: <CAA1CXcCf6=zFw5znf1KE-4tu9q1Am-76DZfQS2XRrUGUBy9QiQ@mail.gmail.com>
Date: Wed, 19 Feb 2025 09:02:46 -0700
From: Nico Pache <npache@...hat.com>
To: Ryan Roberts <ryan.roberts@....com>
Cc: linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org,
linux-mm@...ck.org, 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,
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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 Wed, Feb 19, 2025 at 8:39 AM Ryan Roberts <ryan.roberts@....com> wrote:
>
> 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?
The reasoning was I didn't want to consume a lot of memory for the
mthp_bitmap_stack.
originally the order and offset were u8's, but i had to convert the
offset to u16 to fit the max offset on 64k kernels.
so 64 * (8+16) = 192 bytes as opposed to 1024 bytes if they were ints.
Not sure if using these u8/16 is frowned upon. Lmk if I need to
convert these back to int or if they can stay!
>
> > {
> > 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.
Makes sense, Ill work on adding the new counters for
THP_SCAN_EXCEED_(SWAP_PTE|NONE_PTE|SHARED_PTE). Thanks!
>
> > 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?
Thanks! Ill clean up the nits and add a comment to the swapin function
to describe skipping mTHP swapin.
>
> > 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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