Messages in this thread | | | Date | Wed, 17 Apr 2024 17:02:19 +0200 | Subject | Re: [PATCH 1/1] mm/vmscan: avoid split PMD-mapped THP during shrink_folio_list() | From | David Hildenbrand <> |
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On 17.04.24 16:11, Lance Yang wrote: > When the user no longer requires the pages, they would use madvise(madv_free) > to mark the pages as lazy free. IMO, they would not typically rewrite to the > given range. > > At present, a PMD-mapped THP marked as lazyfree during shrink_folio_list() > is unconditionally split, which may be unnecessary. If the THP is exclusively > mapped and clean, and the PMD associated with it is also clean, then we can > attempt to remove the PMD mapping from it. This change will improve the > efficiency of memory reclamation in this case. > > On an Intel i5 CPU, reclaiming 1GiB of PMD-mapped THPs using > mem_cgroup_force_empty() results in the following runtimes in seconds > (shorter is better): > > -------------------------------------------- > | Old | New | Change | > -------------------------------------------- > | 0.683426 | 0.049197 | -92.80% | > -------------------------------------------- > > Signed-off-by: Lance Yang <ioworker0@gmail.com> > --- > include/linux/huge_mm.h | 1 + > include/linux/rmap.h | 1 + > mm/huge_memory.c | 2 +- > mm/rmap.c | 81 +++++++++++++++++++++++++++++++++++++++++ > mm/vmscan.c | 7 ++++ > 5 files changed, 91 insertions(+), 1 deletion(-) > > diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h > index 7cd07b83a3d0..02a71c05f68a 100644 > --- a/include/linux/huge_mm.h > +++ b/include/linux/huge_mm.h > @@ -36,6 +36,7 @@ bool move_huge_pmd(struct vm_area_struct *vma, unsigned long old_addr, > int change_huge_pmd(struct mmu_gather *tlb, struct vm_area_struct *vma, > pmd_t *pmd, unsigned long addr, pgprot_t newprot, > unsigned long cp_flags); > +inline void zap_deposited_table(struct mm_struct *mm, pmd_t *pmd); > > vm_fault_t vmf_insert_pfn_pmd(struct vm_fault *vmf, pfn_t pfn, bool write); > vm_fault_t vmf_insert_pfn_pud(struct vm_fault *vmf, pfn_t pfn, bool write); > diff --git a/include/linux/rmap.h b/include/linux/rmap.h > index 0f906dc6d280..8c2f45713351 100644 > --- a/include/linux/rmap.h > +++ b/include/linux/rmap.h > @@ -100,6 +100,7 @@ enum ttu_flags { > * do a final flush if necessary */ > TTU_RMAP_LOCKED = 0x80, /* do not grab rmap lock: > * caller holds it */ > + TTU_LAZYFREE_THP = 0x100, /* avoid split PMD-mapped THP */ > }; > > #ifdef CONFIG_MMU > diff --git a/mm/huge_memory.c b/mm/huge_memory.c > index 58f2c4745d80..309fba9624c2 100644 > --- a/mm/huge_memory.c > +++ b/mm/huge_memory.c > @@ -1801,7 +1801,7 @@ bool madvise_free_huge_pmd(struct mmu_gather *tlb, struct vm_area_struct *vma, > return ret; > } > > -static inline void zap_deposited_table(struct mm_struct *mm, pmd_t *pmd) > +inline void zap_deposited_table(struct mm_struct *mm, pmd_t *pmd) > { > pgtable_t pgtable; > > diff --git a/mm/rmap.c b/mm/rmap.c > index 2608c40dffad..4994f9e402d4 100644 > --- a/mm/rmap.c > +++ b/mm/rmap.c > @@ -77,6 +77,7 @@ > #include <linux/mm_inline.h> > > #include <asm/tlbflush.h> > +#include <asm/tlb.h> > > #define CREATE_TRACE_POINTS > #include <trace/events/tlb.h> > @@ -1606,6 +1607,80 @@ void folio_remove_rmap_pmd(struct folio *folio, struct page *page, > #endif > } > > +static bool __try_to_unmap_lazyfree_thp(struct vm_area_struct *vma, > + unsigned long address, > + struct folio *folio) > +{ > + spinlock_t *ptl; > + pmd_t *pmdp, orig_pmd; > + struct mmu_notifier_range range; > + struct mmu_gather tlb; > + struct mm_struct *mm = vma->vm_mm; > + struct page *page; > + bool ret = false; > + > + VM_WARN_ON_FOLIO(!folio_test_anon(folio), folio); > + VM_WARN_ON_FOLIO(folio_test_swapbacked(folio), folio); > + VM_WARN_ON_FOLIO(!folio_test_pmd_mappable(folio), folio); > + VM_WARN_ON_FOLIO(!folio_test_locked(folio), folio); > + > + /* > + * If we encounter a PMD-mapped THP that marked as lazyfree, we > + * will try to unmap it without splitting. > + * > + * The folio exclusively mapped should only have two refs: > + * one from the isolation and one from the rmap. > + */ > + if (folio_entire_mapcount(folio) != 1 || folio_test_dirty(folio) || > + folio_ref_count(folio) != 2)
folio_mapcount() == 1 is a bit nicer. Bit I assume you can drop that completely and only check the refcount?
> + return false; > + > + pmdp = mm_find_pmd(mm, address); > + if (unlikely(!pmdp)) > + return false; > + if (pmd_dirty(*pmdp)) > + return false; > + > + tlb_gather_mmu(&tlb, mm); > + mmu_notifier_range_init(&range, MMU_NOTIFY_CLEAR, 0, mm, > + address & HPAGE_PMD_MASK, > + (address & HPAGE_PMD_MASK) + HPAGE_PMD_SIZE); > + mmu_notifier_invalidate_range_start(&range); > + > + ptl = pmd_lock(mm, pmdp); > + orig_pmd = *pmdp; > + if (unlikely(!pmd_present(orig_pmd) || !pmd_trans_huge(orig_pmd))) > + goto out; > + > + page = pmd_page(orig_pmd); > + if (unlikely(page_folio(page) != folio)) > + goto out; > + > + orig_pmd = pmdp_huge_get_and_clear(mm, address, pmdp); > + tlb_remove_pmd_tlb_entry(&tlb, pmdp, address);
Until this point, the page could have been pinned (including GUP-fast) and we might be in trouble if we drop it.
-- Cheers,
David / dhildenb
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