Re: [PATCH v5 6/6] mm: madvise: Avoid split during MADV_PAGEOUT and MADV_COLD

From: Lance Yang
Date: Tue Apr 02 2024 - 07:30:45 EST

On Tue, Apr 2, 2024 at 7:20 PM Ryan Roberts <ryan.roberts@xxxxxxx> wrote:
>
> On 01/04/2024 13:25, Lance Yang wrote:
> > On Wed, Mar 27, 2024 at 10:46 PM Ryan Roberts <ryan.roberts@armcom> 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@xxxxxxxx>
> >> Signed-off-by: Ryan Roberts <ryan.roberts@xxxxxxx>
> >> ---
> >> 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 8185939df1e8..391f56a1b188 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);
> >
> > IIUC, if the first PTE is a CONT-PTE, then calling ptep_test_and_clear_young()
> > will clear the young bit for the entire contig range to avoid having
> > to unfold. So,
> > the other PTEs within the range don't need to clear again.
> >
> > Maybe we should consider overriding mkold_ptes for arm64?
>
> Yes completely agree. I was saving this for a separate submission though, to
> reduce the complexity of this initial series as much as possible. Let me know if
> you disagree and want to see that change as part of this series.

Feel free to save the change for a separate submission :)

>
> >
> > Thanks,
> > Lance
> >
> >> + 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 eadb79c3a357..efee8e4cd2af 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..bd00b83e7c50 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);
> >> + if (err)
> >> + continue;
> >> + start_pte = pte =
> >> + pte_offset_map_lock(mm, pmd, addr, &ptl);
> >> + if (!start_pte)
> >> + break;
> >> + arch_enter_lazy_mmu_mode();
> >> + 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 9d844582ba38..b5b48f4cf2af 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,
> >> @@ -1553,7 +1553,7 @@ 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,
> >> --
> >> 2.25.1
> >>
>