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Message-ID: <CAA1CXcApRSubSNPt03HHCwciqLK=MOz2c37Cc1zD9c3GJ6CpUA@mail.gmail.com>
Date: Wed, 12 Feb 2025 11:16:43 -0700
From: Nico Pache <npache@...hat.com>
To: Usama Arif <usamaarif642@...il.com>
Cc: linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org,
linux-mm@...ck.org, ryan.roberts@....com, anshuman.khandual@....com,
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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,
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rostedt@...dmis.org, mathieu.desnoyers@...icios.com, tiwai@...e.de,
Johannes Weiner <hannes@...xchg.org>
Subject: Re: [RFC v2 7/9] khugepaged: add mTHP support
On Wed, Feb 12, 2025 at 10:04 AM Usama Arif <usamaarif642@...il.com> wrote:
>
>
>
> On 11/02/2025 00:30, Nico Pache wrote:
> > Introduce the ability for khugepaged to collapse to different mTHP sizes.
> > While scanning a PMD range for potential collapse candidates, keep track
> > of pages in MIN_MTHP_ORDER chunks via a bitmap. Each bit represents a
> > utilized region of order MIN_MTHP_ORDER ptes. We remove the restriction
> > of max_ptes_none during the scan phase so we dont bailout early and miss
> > potential mTHP candidates.
> >
> > After the scan is complete we will perform binary recursion on the
> > bitmap to determine which mTHP size would be most efficient to collapse
> > to. max_ptes_none will be scaled by the attempted collapse order to
> > determine how full a THP must be to be eligible.
> >
> > If a mTHP collapse is attempted, but contains swapped out, or shared
> > pages, we dont perform the collapse.
> >
> > Signed-off-by: Nico Pache <npache@...hat.com>
> > ---
> > mm/khugepaged.c | 122 ++++++++++++++++++++++++++++++++----------------
> > 1 file changed, 83 insertions(+), 39 deletions(-)
> >
> > diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> > index c8048d9ec7fb..cd310989725b 100644
> > --- a/mm/khugepaged.c
> > +++ b/mm/khugepaged.c
> > @@ -1127,13 +1127,14 @@ static int collapse_huge_page(struct mm_struct *mm, unsigned long address,
> > {
> > LIST_HEAD(compound_pagelist);
> > pmd_t *pmd, _pmd;
> > - pte_t *pte;
> > + pte_t *pte, mthp_pte;
> > pgtable_t pgtable;
> > struct folio *folio;
> > spinlock_t *pmd_ptl, *pte_ptl;
> > int result = SCAN_FAIL;
> > struct vm_area_struct *vma;
> > struct mmu_notifier_range range;
> > + unsigned long _address = address + offset * PAGE_SIZE;
> > VM_BUG_ON(address & ~HPAGE_PMD_MASK);
> >
> > /*
> > @@ -1148,12 +1149,13 @@ static int collapse_huge_page(struct mm_struct *mm, unsigned long address,
> > *mmap_locked = false;
> > }
> >
> > - result = alloc_charge_folio(&folio, mm, cc, HPAGE_PMD_ORDER);
> > + result = alloc_charge_folio(&folio, mm, cc, order);
> > if (result != SCAN_SUCCEED)
> > goto out_nolock;
> >
> > mmap_read_lock(mm);
> > - result = hugepage_vma_revalidate(mm, address, true, &vma, cc, HPAGE_PMD_ORDER);
> > + *mmap_locked = true;
> > + result = hugepage_vma_revalidate(mm, address, true, &vma, cc, order);
> > if (result != SCAN_SUCCEED) {
> > mmap_read_unlock(mm);
> > goto out_nolock;
> > @@ -1171,13 +1173,14 @@ static int collapse_huge_page(struct mm_struct *mm, unsigned long address,
> > * released when it fails. So we jump out_nolock directly in
> > * that case. Continuing to collapse causes inconsistency.
> > */
> > - result = __collapse_huge_page_swapin(mm, vma, address, pmd,
> > - referenced, HPAGE_PMD_ORDER);
> > + result = __collapse_huge_page_swapin(mm, vma, _address, pmd,
> > + referenced, order);
> > if (result != SCAN_SUCCEED)
> > goto out_nolock;
> > }
> >
> > mmap_read_unlock(mm);
> > + *mmap_locked = false;
> > /*
> > * Prevent all access to pagetables with the exception of
> > * gup_fast later handled by the ptep_clear_flush and the VM
> > @@ -1187,7 +1190,7 @@ static int collapse_huge_page(struct mm_struct *mm, unsigned long address,
> > * mmap_lock.
> > */
> > mmap_write_lock(mm);
> > - result = hugepage_vma_revalidate(mm, address, true, &vma, cc, HPAGE_PMD_ORDER);
> > + result = hugepage_vma_revalidate(mm, address, true, &vma, cc, order);
> > if (result != SCAN_SUCCEED)
> > goto out_up_write;
> > /* check if the pmd is still valid */
> > @@ -1198,11 +1201,12 @@ static int collapse_huge_page(struct mm_struct *mm, unsigned long address,
> > vma_start_write(vma);
> > anon_vma_lock_write(vma->anon_vma);
> >
> > - mmu_notifier_range_init(&range, MMU_NOTIFY_CLEAR, 0, mm, address,
> > - address + HPAGE_PMD_SIZE);
> > + mmu_notifier_range_init(&range, MMU_NOTIFY_CLEAR, 0, mm, _address,
> > + _address + (PAGE_SIZE << order));
> > mmu_notifier_invalidate_range_start(&range);
> >
> > pmd_ptl = pmd_lock(mm, pmd); /* probably unnecessary */
> > +
> > /*
> > * This removes any huge TLB entry from the CPU so we won't allow
> > * huge and small TLB entries for the same virtual address to
> > @@ -1216,10 +1220,10 @@ static int collapse_huge_page(struct mm_struct *mm, unsigned long address,
> > mmu_notifier_invalidate_range_end(&range);
> > tlb_remove_table_sync_one();
> >
> > - pte = pte_offset_map_lock(mm, &_pmd, address, &pte_ptl);
> > + pte = pte_offset_map_lock(mm, &_pmd, _address, &pte_ptl);
> > if (pte) {
> > - result = __collapse_huge_page_isolate(vma, address, pte, cc,
> > - &compound_pagelist, HPAGE_PMD_ORDER);
> > + result = __collapse_huge_page_isolate(vma, _address, pte, cc,
> > + &compound_pagelist, order);
> > spin_unlock(pte_ptl);
> > } else {
> > result = SCAN_PMD_NULL;
> > @@ -1248,8 +1252,8 @@ static int collapse_huge_page(struct mm_struct *mm, unsigned long address,
> > anon_vma_unlock_write(vma->anon_vma);
> >
> > result = __collapse_huge_page_copy(pte, folio, pmd, _pmd,
> > - vma, address, pte_ptl,
> > - &compound_pagelist, HPAGE_PMD_ORDER);
> > + vma, _address, pte_ptl,
> > + &compound_pagelist, order);
> > pte_unmap(pte);
> > if (unlikely(result != SCAN_SUCCEED))
> > goto out_up_write;
> > @@ -1260,20 +1264,37 @@ static int collapse_huge_page(struct mm_struct *mm, unsigned long address,
> > * write.
> > */
> > __folio_mark_uptodate(folio);
> > - pgtable = pmd_pgtable(_pmd);
> > -
> > - _pmd = mk_huge_pmd(&folio->page, vma->vm_page_prot);
> > - _pmd = maybe_pmd_mkwrite(pmd_mkdirty(_pmd), vma);
> > -
> > - spin_lock(pmd_ptl);
> > - BUG_ON(!pmd_none(*pmd));
> > - folio_add_new_anon_rmap(folio, vma, address, RMAP_EXCLUSIVE);
> > - folio_add_lru_vma(folio, vma);
> > - pgtable_trans_huge_deposit(mm, pmd, pgtable);
> > - set_pmd_at(mm, address, pmd, _pmd);
> > - update_mmu_cache_pmd(vma, address, pmd);
> > - deferred_split_folio(folio, false);
> > - spin_unlock(pmd_ptl);
> > + if (order == HPAGE_PMD_ORDER) {
> > + pgtable = pmd_pgtable(_pmd);
> > + _pmd = mk_huge_pmd(&folio->page, vma->vm_page_prot);
> > + _pmd = maybe_pmd_mkwrite(pmd_mkdirty(_pmd), vma);
> > +
> > + spin_lock(pmd_ptl);
> > + BUG_ON(!pmd_none(*pmd));
> > + folio_add_new_anon_rmap(folio, vma, _address, RMAP_EXCLUSIVE);
> > + folio_add_lru_vma(folio, vma);
> > + pgtable_trans_huge_deposit(mm, pmd, pgtable);
> > + set_pmd_at(mm, address, pmd, _pmd);
> > + update_mmu_cache_pmd(vma, address, pmd);
> > + deferred_split_folio(folio, false);
> > + spin_unlock(pmd_ptl);
> > + } else { //mTHP
> > + mthp_pte = mk_pte(&folio->page, vma->vm_page_prot);
> > + mthp_pte = maybe_mkwrite(pte_mkdirty(mthp_pte), vma);
> > +
> > + spin_lock(pmd_ptl);
> > + folio_ref_add(folio, (1 << order) - 1);
> > + folio_add_new_anon_rmap(folio, vma, _address, RMAP_EXCLUSIVE);
> > + folio_add_lru_vma(folio, vma);
> > + spin_lock(pte_ptl);
> > + set_ptes(vma->vm_mm, _address, pte, mthp_pte, (1 << order));
> > + update_mmu_cache_range(NULL, vma, _address, pte, (1 << order));
> > + spin_unlock(pte_ptl);
> > + smp_wmb(); /* make pte visible before pmd */
> > + pmd_populate(mm, pmd, pmd_pgtable(_pmd));
> > + deferred_split_folio(folio, false);
>
>
> Hi Nico,
>
> This patch will have the same issue as the one I pointed out in
> https://lore.kernel.org/all/82b9efd1-f2a6-4452-b2ea-6c163e17cdf7@gmail.com/ ?
Hi Usama,
Yes thanks for pointing that out! I'll make sure to remove the
deferred_split_folio call.
>
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