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Date: Tue, 14 May 2024 15:09:43 +0800
From: Baolin Wang <baolin.wang@...ux.alibaba.com>
To: Lance Yang <ioworker0@...il.com>, akpm@...ux-foundation.org
Cc: 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()
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.
> +
> + tlb_gather_mmu(&tlb, mm);
You missed tlb_finish_mmu() to do tlb flushing, and ...
> + 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.
> +
> + /*
> + * 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?
> + 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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