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Date: Wed, 8 May 2024 13:14:27 +0800
From: Lance Yang <ioworker0@...il.com>
To: Zi Yan <ziy@...dia.com>
Cc: akpm@...ux-foundation.org, willy@...radead.org, sj@...nel.org,
maskray@...gle.com, ryan.roberts@....com, david@...hat.com, 21cnbao@...il.com,
mhocko@...e.com, fengwei.yin@...el.com, zokeefe@...gle.com,
shy828301@...il.com, xiehuan09@...il.com, libang.li@...group.com,
wangkefeng.wang@...wei.com, songmuchun@...edance.com, peterx@...hat.com,
minchan@...nel.org, linux-mm@...ck.org, linux-kernel@...r.kernel.org,
Baolin Wang <baolin.wang@...ux.alibaba.com>
Subject: Re: [PATCH v4 3/3] mm/vmscan: avoid split lazyfree THP during shrink_folio_list()
Hey Zi,
Thanks for taking time to review!
On Wed, May 8, 2024 at 12:20 AM Zi Yan <ziy@...dia.com> wrote:
>
> On 1 May 2024, at 0:27, Lance Yang wrote:
>
> > When the user no longer requires the pages, they would use
> > madvise(MADV_FREE) to mark the pages as lazy free. Subsequently, they
> > typically would not re-write to that memory again.
> >
> > During memory reclaim, if we detect that the large folio and its PMD are
> > both still marked as clean and there are no unexpected references
> > (such as GUP), so we can just discard the memory lazily, improving the
> > efficiency of memory reclamation in this case. On an Intel i5 CPU, reclaiming 1GiB of lazyfree 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% |
> > --------------------------------------------
> >
> > Suggested-by: Zi Yan <ziy@...dia.com>
> > Suggested-by: David Hildenbrand <david@...hat.com>
> > Signed-off-by: Lance Yang <ioworker0@...il.com>
> > ---
> > include/linux/huge_mm.h | 9 +++++
> > mm/huge_memory.c | 73 +++++++++++++++++++++++++++++++++++++++++
> > mm/rmap.c | 3 ++
> > 3 files changed, 85 insertions(+)
> >
> > diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
> > index 38c4b5537715..017cee864080 100644
> > --- a/include/linux/huge_mm.h
> > +++ b/include/linux/huge_mm.h
> > @@ -411,6 +411,8 @@ static inline bool thp_migration_supported(void)
> >
> > void split_huge_pmd_locked(struct vm_area_struct *vma, unsigned long address,
> > pmd_t *pmd, bool freeze, struct folio *folio);
> > +bool unmap_huge_pmd_locked(struct vm_area_struct *vma, unsigned long addr,
> > + pmd_t *pmdp, struct folio *folio);
> >
> > static inline void align_huge_pmd_range(struct vm_area_struct *vma,
> > unsigned long *start,
> > @@ -492,6 +494,13 @@ static inline void align_huge_pmd_range(struct vm_area_struct *vma,
> > unsigned long *start,
> > unsigned long *end) {}
> >
> > +static inline bool unmap_huge_pmd_locked(struct vm_area_struct *vma,
> > + unsigned long addr, pmd_t *pmdp,
> > + struct folio *folio)
> > +{
> > + return false;
> > +}
> > +
> > #define split_huge_pud(__vma, __pmd, __address) \
> > do { } while (0)
> >
> > diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> > index 145505a1dd05..90fdef847a88 100644
> > --- a/mm/huge_memory.c
> > +++ b/mm/huge_memory.c
> > @@ -2690,6 +2690,79 @@ static void unmap_folio(struct folio *folio)
> > try_to_unmap_flush();
> > }
> >
> > +static bool __discard_trans_pmd_locked(struct vm_area_struct *vma,
> > + unsigned long addr, pmd_t *pmdp,
> > + struct folio *folio)
> > +{
> > + struct mm_struct *mm = vma->vm_mm;
> > + int ref_count, map_count;
> > + pmd_t orig_pmd = *pmdp;
> > + struct mmu_gather tlb;
> > + struct page *page;
> > +
> > + if (pmd_dirty(orig_pmd) || folio_test_dirty(folio))
> > + return false;
> > + if (unlikely(!pmd_present(orig_pmd) || !pmd_trans_huge(orig_pmd)))
> > + return false;
> > +
> > + page = pmd_page(orig_pmd);
> > + if (unlikely(page_folio(page) != folio))
> > + return false;
> > +
> > + tlb_gather_mmu(&tlb, mm);
> > + orig_pmd = pmdp_huge_get_and_clear(mm, addr, pmdp);
> > + tlb_remove_pmd_tlb_entry(&tlb, pmdp, addr);
> > +
> > + /*
> > + * Syncing against concurrent GUP-fast:
> > + * - clear PMD; barrier; read refcount
> > + * - inc refcount; barrier; read PMD
> > + */
> > + smp_mb();
> > +
> > + ref_count = folio_ref_count(folio);
> > + map_count = folio_mapcount(folio);
> > +
> > + /*
> > + * Order reads for folio refcount and dirty flag
> > + * (see comments in __remove_mapping()).
> > + */
> > + smp_rmb();
> > +
> > + /*
> > + * If the PMD or folio is redirtied at this point, or if there are
> > + * unexpected references, we will give up to discard this folio
> > + * and remap it.
> > + *
> > + * The only folio refs must be one from isolation plus the rmap(s).
> > + */
> > + if (ref_count != map_count + 1 || folio_test_dirty(folio) ||
> > + pmd_dirty(orig_pmd)) {
> > + set_pmd_at(mm, addr, pmdp, orig_pmd);
> > + return false;
> > + }
> > +
> > + folio_remove_rmap_pmd(folio, page, vma);
> > + zap_deposited_table(mm, pmdp);
> > + add_mm_counter(mm, MM_ANONPAGES, -HPAGE_PMD_NR);
> > + folio_put(folio);
> > +
> > + return true;
> > +}
> > +
> > +bool unmap_huge_pmd_locked(struct vm_area_struct *vma, unsigned long addr,
> > + pmd_t *pmdp, struct folio *folio)
> > +{
> > + VM_WARN_ON_FOLIO(!folio_test_pmd_mappable(folio), folio);
> > + VM_WARN_ON_FOLIO(!folio_test_locked(folio), folio);
> > + VM_WARN_ON_ONCE(!IS_ALIGNED(addr, HPAGE_PMD_SIZE));
> > +
> > + if (folio_test_anon(folio) && !folio_test_swapbacked(folio))
> > + return __discard_trans_pmd_locked(vma, addr, pmdp, folio);
> > +
> > + return false;
> > +}
> > +
> > static void remap_page(struct folio *folio, unsigned long nr)
> > {
> > int i = 0;
> > diff --git a/mm/rmap.c b/mm/rmap.c
> > index 432601154583..1d3d30cb752c 100644
> > --- a/mm/rmap.c
> > +++ b/mm/rmap.c
> > @@ -1675,6 +1675,9 @@ static bool try_to_unmap_one(struct folio *folio, struct vm_area_struct *vma,
> > }
> >
> > if (!pvmw.pte && (flags & TTU_SPLIT_HUGE_PMD)) {
> > + if (unmap_huge_pmd_locked(vma, range.start, pvmw.pmd,
> > + folio))
> > + goto walk_done;
>
> You might not need to check (flags & TTU_SPLIT_HUGE_PMD) for
> unmap_huge_pmd_locked(), since you are unmapping a PMD here.
> TTU_SPLIT_HUGE_PMD is here because try_to_unmap_one() was not able to unmap
> a PMD. You probably can remove it for callers that are unmapping
> the folio but not the ones are swapping.
Thanks for the suggestion!
Ageed. For unmap_huge_pmd_locked(), there is no need to check the
TTU_SPLIT_HUGE_PMD flag. We only need to check the flag for
split_huge_pmd_locked().
Given this, if we fail to remove the PMD mapping and the flag is not set,
I think we should stop the walk. So we can also remove the
VM_BUG_ON_FOLIO() below.
/* Unexpected PMD-mapped THP? */
VM_BUG_ON_FOLIO(!pvmw.pte, folio);
Zi, what do you think?
Thanks,
Lance
>
>
>
> > /*
> > * We temporarily have to drop the PTL and start once
> > * again from that now-PTE-mapped page table.
> > --
> > 2.33.1
>
>
> --
> Best Regards,
> Yan, Zi
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