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Date: Tue, 14 May 2024 16:36:42 +0800
From: Lance Yang <ioworker0@...il.com>
To: Baolin Wang <baolin.wang@...ux.alibaba.com>
Cc: akpm@...ux-foundation.org, willy@...radead.org, sj@...nel.org,
maskray@...gle.com, ziy@...dia.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
Subject: Re: [PATCH RESEND v5 4/4] mm/vmscan: avoid split lazyfree THP during shrink_folio_list()
Hi Baolin,
Thanks for taking time to review!
On Tue, May 14, 2024 at 3:09 PM Baolin Wang
<baolin.wang@...ux.alibaba.com> wrote:
>
>
>
> On 2024/5/13 15:47, 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 | 75 +++++++++++++++++++++++++++++++++++++++++
> > mm/rmap.c | 31 ++++++++++-------
> > 3 files changed, 103 insertions(+), 12 deletions(-)
> >
> > diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
> > index 9fcb0b0b6ed1..cfd7ec2b6d0a 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);
> >
> > #else /* CONFIG_TRANSPARENT_HUGEPAGE */
> >
> > @@ -478,6 +480,13 @@ static inline void split_huge_pmd_locked(struct vm_area_struct *vma,
> > unsigned long address, pmd_t *pmd,
> > bool freeze, struct folio *folio) {}
> >
> > +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 425272c6c50b..3ceeeb2f42d4 100644
> > --- a/mm/huge_memory.c
> > +++ b/mm/huge_memory.c
> > @@ -2687,6 +2687,81 @@ 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;
>
> The function is called under the ptl lock, so I have no idea why the pmd
> value can be changed, seems above validation is useless.
Yep. For the page reclaim path, page_vma_mapped_walk() will require
the PTL, and make sure the PMD mapping is unchanged via check_pmd().
But, IMO, we cannot assume that all callers always do the PMD mapping
check. I suggest keeping it to prepare for corner cases :)
>
> > +
> > + tlb_gather_mmu(&tlb, mm);
>
> You missed tlb_finish_mmu() to do tlb flushing, and ...
Good spot!
>
> > + orig_pmd = pmdp_huge_get_and_clear(mm, addr, pmdp);
> > + tlb_remove_pmd_tlb_entry(&tlb, pmdp, addr);
>
> I don't think tlb gather is helpful here, since you just flush one PMD
> entry. Just using pmdp_huge_clear_flush() seems enough.
Nice, thanks for the suggestion!
I completely agree that using pmdp_huge_clear_flush() is sufficient here :)
>
> > +
> > + /*
> > + * 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);
>
> Should we also call 'folio_set_swapbacked()' if the folio was redirtied?
Yes. I forgot about that :)
Thanks again for the review!
Lance
>
> > + return false;
> > + }
> > +
> > + folio_remove_rmap_pmd(folio, page, vma);
> > + zap_deposited_table(mm, pmdp);
> > + add_mm_counter(mm, MM_ANONPAGES, -HPAGE_PMD_NR);
> > + if (vma->vm_flags & VM_LOCKED)
> > + mlock_drain_local();
> > + 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 08a93347f283..e09f2141b8dc 100644
> > --- a/mm/rmap.c
> > +++ b/mm/rmap.c
> > @@ -1677,18 +1677,25 @@ static bool try_to_unmap_one(struct folio *folio, struct vm_area_struct *vma,
> > goto walk_done_err;
> > }
> >
> > - if (!pvmw.pte && (flags & TTU_SPLIT_HUGE_PMD)) {
> > - /*
> > - * We temporarily have to drop the PTL and start once
> > - * again from that now-PTE-mapped page table.
> > - */
> > - split_huge_pmd_locked(vma, range.start, pvmw.pmd, false,
> > - folio);
> > - pvmw.pmd = NULL;
> > - spin_unlock(pvmw.ptl);
> > - pvmw.ptl = NULL;
> > - flags &= ~TTU_SPLIT_HUGE_PMD;
> > - continue;
> > + if (!pvmw.pte) {
> > + if (unmap_huge_pmd_locked(vma, range.start, pvmw.pmd,
> > + folio))
> > + goto walk_done;
> > +
> > + if (flags & TTU_SPLIT_HUGE_PMD) {
> > + /*
> > + * We temporarily have to drop the PTL and start
> > + * once again from that now-PTE-mapped page
> > + * table.
> > + */
> > + split_huge_pmd_locked(vma, range.start,
> > + pvmw.pmd, false, folio);
> > + pvmw.pmd = NULL;
> > + spin_unlock(pvmw.ptl);
> > + pvmw.ptl = NULL;
> > + flags &= ~TTU_SPLIT_HUGE_PMD;
> > + continue;
> > + }
> > }
> >
> > /* Unexpected PMD-mapped THP? */
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