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Message-ID: <5c42d627-d601-4397-823a-e3e113a00ff7@arm.com>
Date: Wed, 3 Apr 2024 18:17:56 +0100
From: Ryan Roberts <ryan.roberts@....com>
To: Andrew Morton <akpm@...ux-foundation.org>,
David Hildenbrand <david@...hat.com>, Matthew Wilcox <willy@...radead.org>,
Huang Ying <ying.huang@...el.com>, Gao Xiang <xiang@...nel.org>,
Yu Zhao <yuzhao@...gle.com>, Yang Shi <shy828301@...il.com>,
Michal Hocko <mhocko@...e.com>, Kefeng Wang <wangkefeng.wang@...wei.com>,
Barry Song <21cnbao@...il.com>, Chris Li <chrisl@...nel.org>,
Lance Yang <ioworker0@...il.com>
Cc: linux-mm@...ck.org, linux-kernel@...r.kernel.org,
Barry Song <v-songbaohua@...o.com>
Subject: Re: [PATCH v6 6/6] mm: madvise: Avoid split during MADV_PAGEOUT and
MADV_COLD
On 03/04/2024 12:40, Ryan Roberts wrote:
> Rework madvise_cold_or_pageout_pte_range() to avoid splitting any large
> folio that is fully and contiguously mapped in the pageout/cold vm
> range. This change means that large folios will be maintained all the
> way to swap storage. This both improves performance during swap-out, by
> eliding the cost of splitting the folio, and sets us up nicely for
> maintaining the large folio when it is swapped back in (to be covered in
> a separate series).
>
> Folios that are not fully mapped in the target range are still split,
> but note that behavior is changed so that if the split fails for any
> reason (folio locked, shared, etc) we now leave it as is and move to the
> next pte in the range and continue work on the proceeding folios.
> Previously any failure of this sort would cause the entire operation to
> give up and no folios mapped at higher addresses were paged out or made
> cold. Given large folios are becoming more common, this old behavior
> would have likely lead to wasted opportunities.
>
> While we are at it, change the code that clears young from the ptes to
> use ptep_test_and_clear_young(), via the new mkold_ptes() batch helper
> function. This is more efficent than get_and_clear/modify/set,
> especially for contpte mappings on arm64, where the old approach would
> require unfolding/refolding and the new approach can be done in place.
>
> Reviewed-by: Barry Song <v-songbaohua@...o.com>
> Signed-off-by: Ryan Roberts <ryan.roberts@....com>
> ---
> include/linux/pgtable.h | 30 ++++++++++++++
> mm/internal.h | 12 +++++-
> mm/madvise.c | 88 ++++++++++++++++++++++++-----------------
> mm/memory.c | 4 +-
> 4 files changed, 93 insertions(+), 41 deletions(-)
>
> diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
> index 0278259f7078..0f4b2faa1d71 100644
> --- a/include/linux/pgtable.h
> +++ b/include/linux/pgtable.h
> @@ -361,6 +361,36 @@ static inline int ptep_test_and_clear_young(struct vm_area_struct *vma,
> }
> #endif
>
> +#ifndef mkold_ptes
> +/**
> + * mkold_ptes - Mark PTEs that map consecutive pages of the same folio as old.
> + * @vma: VMA the pages are mapped into.
> + * @addr: Address the first page is mapped at.
> + * @ptep: Page table pointer for the first entry.
> + * @nr: Number of entries to mark old.
> + *
> + * May be overridden by the architecture; otherwise, implemented as a simple
> + * loop over ptep_test_and_clear_young().
> + *
> + * Note that PTE bits in the PTE range besides the PFN can differ. For example,
> + * some PTEs might be write-protected.
> + *
> + * Context: The caller holds the page table lock. The PTEs map consecutive
> + * pages that belong to the same folio. The PTEs are all in the same PMD.
> + */
> +static inline void mkold_ptes(struct vm_area_struct *vma, unsigned long addr,
> + pte_t *ptep, unsigned int nr)
> +{
> + for (;;) {
> + ptep_test_and_clear_young(vma, addr, ptep);
> + if (--nr == 0)
> + break;
> + ptep++;
> + addr += PAGE_SIZE;
> + }
> +}
> +#endif
> +
> #ifndef __HAVE_ARCH_PMDP_TEST_AND_CLEAR_YOUNG
> #if defined(CONFIG_TRANSPARENT_HUGEPAGE) || defined(CONFIG_ARCH_HAS_NONLEAF_PMD_YOUNG)
> static inline int pmdp_test_and_clear_young(struct vm_area_struct *vma,
> diff --git a/mm/internal.h b/mm/internal.h
> index 88705ab4c50a..003bc189736b 100644
> --- a/mm/internal.h
> +++ b/mm/internal.h
> @@ -130,6 +130,8 @@ static inline pte_t __pte_batch_clear_ignored(pte_t pte, fpb_t flags)
> * @flags: Flags to modify the PTE batch semantics.
> * @any_writable: Optional pointer to indicate whether any entry except the
> * first one is writable.
> + * @any_young: Optional pointer to indicate whether any entry except the
> + * first one is young.
> *
> * Detect a PTE batch: consecutive (present) PTEs that map consecutive
> * pages of the same large folio.
> @@ -145,16 +147,18 @@ static inline pte_t __pte_batch_clear_ignored(pte_t pte, fpb_t flags)
> */
> static inline int folio_pte_batch(struct folio *folio, unsigned long addr,
> pte_t *start_ptep, pte_t pte, int max_nr, fpb_t flags,
> - bool *any_writable)
> + bool *any_writable, bool *any_young)
> {
> unsigned long folio_end_pfn = folio_pfn(folio) + folio_nr_pages(folio);
> const pte_t *end_ptep = start_ptep + max_nr;
> pte_t expected_pte, *ptep;
> - bool writable;
> + bool writable, young;
> int nr;
>
> if (any_writable)
> *any_writable = false;
> + if (any_young)
> + *any_young = false;
>
> VM_WARN_ON_FOLIO(!pte_present(pte), folio);
> VM_WARN_ON_FOLIO(!folio_test_large(folio) || max_nr < 1, folio);
> @@ -168,6 +172,8 @@ static inline int folio_pte_batch(struct folio *folio, unsigned long addr,
> pte = ptep_get(ptep);
> if (any_writable)
> writable = !!pte_write(pte);
> + if (any_young)
> + young = !!pte_young(pte);
> pte = __pte_batch_clear_ignored(pte, flags);
>
> if (!pte_same(pte, expected_pte))
> @@ -183,6 +189,8 @@ static inline int folio_pte_batch(struct folio *folio, unsigned long addr,
>
> if (any_writable)
> *any_writable |= writable;
> + if (any_young)
> + *any_young |= young;
>
> nr = pte_batch_hint(ptep, pte);
> expected_pte = pte_advance_pfn(expected_pte, nr);
> diff --git a/mm/madvise.c b/mm/madvise.c
> index 070bedb4996e..c1377f3b5ca1 100644
> --- a/mm/madvise.c
> +++ b/mm/madvise.c
> @@ -336,6 +336,7 @@ static int madvise_cold_or_pageout_pte_range(pmd_t *pmd,
> LIST_HEAD(folio_list);
> bool pageout_anon_only_filter;
> unsigned int batch_count = 0;
> + int nr;
>
> if (fatal_signal_pending(current))
> return -EINTR;
> @@ -423,7 +424,8 @@ static int madvise_cold_or_pageout_pte_range(pmd_t *pmd,
> return 0;
> flush_tlb_batched_pending(mm);
> arch_enter_lazy_mmu_mode();
> - for (; addr < end; pte++, addr += PAGE_SIZE) {
> + for (; addr < end; pte += nr, addr += nr * PAGE_SIZE) {
> + nr = 1;
> ptent = ptep_get(pte);
>
> if (++batch_count == SWAP_CLUSTER_MAX) {
> @@ -447,55 +449,67 @@ static int madvise_cold_or_pageout_pte_range(pmd_t *pmd,
> continue;
>
> /*
> - * Creating a THP page is expensive so split it only if we
> - * are sure it's worth. Split it if we are only owner.
> + * If we encounter a large folio, only split it if it is not
> + * fully mapped within the range we are operating on. Otherwise
> + * leave it as is so that it can be swapped out whole. If we
> + * fail to split a folio, leave it in place and advance to the
> + * next pte in the range.
> */
> if (folio_test_large(folio)) {
> - int err;
> -
> - if (folio_likely_mapped_shared(folio))
> - break;
> - if (pageout_anon_only_filter && !folio_test_anon(folio))
> - break;
> - if (!folio_trylock(folio))
> - break;
> - folio_get(folio);
> - arch_leave_lazy_mmu_mode();
> - pte_unmap_unlock(start_pte, ptl);
> - start_pte = NULL;
> - err = split_folio(folio);
> - folio_unlock(folio);
> - folio_put(folio);
> - if (err)
> - break;
> - start_pte = pte =
> - pte_offset_map_lock(mm, pmd, addr, &ptl);
> - if (!start_pte)
> - break;
> - arch_enter_lazy_mmu_mode();
> - pte--;
> - addr -= PAGE_SIZE;
> - continue;
> + const fpb_t fpb_flags = FPB_IGNORE_DIRTY |
> + FPB_IGNORE_SOFT_DIRTY;
> + int max_nr = (end - addr) / PAGE_SIZE;
> + bool any_young;
> +
> + nr = folio_pte_batch(folio, addr, pte, ptent, max_nr,
> + fpb_flags, NULL, &any_young);
> + if (any_young)
> + ptent = pte_mkyoung(ptent);
> +
> + if (nr < folio_nr_pages(folio)) {
> + int err;
> +
> + if (folio_likely_mapped_shared(folio))
> + continue;
> + if (pageout_anon_only_filter && !folio_test_anon(folio))
> + continue;
> + if (!folio_trylock(folio))
> + continue;
> + folio_get(folio);
> + arch_leave_lazy_mmu_mode();
> + pte_unmap_unlock(start_pte, ptl);
> + start_pte = NULL;
> + err = split_folio(folio);
> + folio_unlock(folio);
> + folio_put(folio);
> + start_pte = pte =
> + pte_offset_map_lock(mm, pmd, addr, &ptl);
> + if (!start_pte)
> + break;
> + if (err)
> + continue;
> + arch_enter_lazy_mmu_mode();
Damn, this is still broken; arch_enter_lazy_mmu_mode() needs to be before the
continue. (thanks to Lance Yang; I noticed this while reviewing his MADV_FREE
series).
Andrew, is it possible to fix this up as you add it to mm-unstable, or do you
want me to send a v7?
The change would be:
diff --git a/mm/madvise.c b/mm/madvise.c
index c1377f3b5ca1..bf26cf2b7715 100644
--- a/mm/madvise.c
+++ b/mm/madvise.c
@@ -486,10 +486,9 @@ static int madvise_cold_or_pageout_pte_range(pmd_t *pmd,
pte_offset_map_lock(mm, pmd, addr, &ptl);
if (!start_pte)
break;
- if (err)
- continue;
arch_enter_lazy_mmu_mode();
- nr = 0;
+ if (!err)
+ nr = 0;
continue;
}
}
> + nr = 0;
> + continue;
> + }
> }
>
> /*
> * Do not interfere with other mappings of this folio and
> - * non-LRU folio.
> + * non-LRU folio. If we have a large folio at this point, we
> + * know it is fully mapped so if its mapcount is the same as its
> + * number of pages, it must be exclusive.
> */
> - if (!folio_test_lru(folio) || folio_mapcount(folio) != 1)
> + if (!folio_test_lru(folio) ||
> + folio_mapcount(folio) != folio_nr_pages(folio))
> continue;
>
> if (pageout_anon_only_filter && !folio_test_anon(folio))
> continue;
>
> - VM_BUG_ON_FOLIO(folio_test_large(folio), folio);
> -
> if (!pageout && pte_young(ptent)) {
> - ptent = ptep_get_and_clear_full(mm, addr, pte,
> - tlb->fullmm);
> - ptent = pte_mkold(ptent);
> - set_pte_at(mm, addr, pte, ptent);
> - tlb_remove_tlb_entry(tlb, pte, addr);
> + mkold_ptes(vma, addr, pte, nr);
> + tlb_remove_tlb_entries(tlb, pte, nr, addr);
> }
>
> /*
> diff --git a/mm/memory.c b/mm/memory.c
> index ef2968894718..912cd738ec03 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -989,7 +989,7 @@ copy_present_ptes(struct vm_area_struct *dst_vma, struct vm_area_struct *src_vma
> flags |= FPB_IGNORE_SOFT_DIRTY;
>
> nr = folio_pte_batch(folio, addr, src_pte, pte, max_nr, flags,
> - &any_writable);
> + &any_writable, NULL);
> folio_ref_add(folio, nr);
> if (folio_test_anon(folio)) {
> if (unlikely(folio_try_dup_anon_rmap_ptes(folio, page,
> @@ -1559,7 +1559,7 @@ static inline int zap_present_ptes(struct mmu_gather *tlb,
> */
> if (unlikely(folio_test_large(folio) && max_nr != 1)) {
> nr = folio_pte_batch(folio, addr, pte, ptent, max_nr, fpb_flags,
> - NULL);
> + NULL, NULL);
>
> zap_present_folio_ptes(tlb, vma, folio, page, pte, ptent, nr,
> addr, details, rss, force_flush,
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