Re: [PATCH v8 3/3] mm/madvise: optimize lazyfreeing with mTHP in madvise_free

From: Ryan Roberts
Date: Wed Apr 17 2024 - 12:25:28 EST

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On 17/04/2024 15:14, Lance Yang wrote:
> This patch optimizes lazyfreeing with PTE-mapped mTHP[1]
> (Inspired by David Hildenbrand[2]). We aim to avoid unnecessary folio
> splitting if the large folio is fully mapped within the target range.
>
> If a large folio is locked or shared, or if we fail to split it, we just
> leave it in place and advance to the next PTE in the range. But note that
> the behavior is changed; previously, any failure of this sort would cause
> the entire operation to give up. As large folios become more common,
> sticking to the old way could result in wasted opportunities.
>
> On an Intel I5 CPU, lazyfreeing a 1GiB VMA backed by PTE-mapped folios of
> the same size results in the following runtimes for madvise(MADV_FREE) in
> seconds (shorter is better):
>
> Folio Size | Old | New | Change
> ------------------------------------------
> 4KiB | 0.590251 | 0.590259 | 0%
> 16KiB | 2.990447 | 0.185655 | -94%
> 32KiB | 2.547831 | 0.104870 | -95%
> 64KiB | 2.457796 | 0.052812 | -97%
> 128KiB | 2.281034 | 0.032777 | -99%
> 256KiB | 2.230387 | 0.017496 | -99%
> 512KiB | 2.189106 | 0.010781 | -99%
> 1024KiB | 2.183949 | 0.007753 | -99%
> 2048KiB | 0.002799 | 0.002804 | 0%
>
> [1] https://lkml.kernel.org/r/20231207161211.2374093-5-ryan.roberts@xxxxxxx
> [2] https://lore.kernel.org/linux-mm/20240214204435.167852-1-david@xxxxxxxxxx
>
> Signed-off-by: Lance Yang <ioworker0@xxxxxxxxx>

LGTM!

Reviewed-by: Ryan Roberts <ryan.roberts@xxxxxxx>

> ---
> mm/internal.h | 12 ++++-
> mm/madvise.c | 141 ++++++++++++++++++++++++++++----------------------
> mm/memory.c | 4 +-
> 3 files changed, 91 insertions(+), 66 deletions(-)
>
> diff --git a/mm/internal.h b/mm/internal.h
> index c6483f73ec13..daa59cef85d7 100644
> --- a/mm/internal.h
> +++ b/mm/internal.h
> @@ -134,6 +134,8 @@ static inline pte_t __pte_batch_clear_ignored(pte_t pte, fpb_t flags)
> * first one is writable.
> * @any_young: Optional pointer to indicate whether any entry except the
> * first one is young.
> + * @any_dirty: Optional pointer to indicate whether any entry except the
> + * first one is dirty.
> *
> * Detect a PTE batch: consecutive (present) PTEs that map consecutive
> * pages of the same large folio.
> @@ -149,18 +151,20 @@ 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_young)
> + bool *any_writable, bool *any_young, bool *any_dirty)
> {
> 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, young;
> + bool writable, young, dirty;
> int nr;
>
> if (any_writable)
> *any_writable = false;
> if (any_young)
> *any_young = false;
> + if (any_dirty)
> + *any_dirty = false;
>
> VM_WARN_ON_FOLIO(!pte_present(pte), folio);
> VM_WARN_ON_FOLIO(!folio_test_large(folio) || max_nr < 1, folio);
> @@ -176,6 +180,8 @@ static inline int folio_pte_batch(struct folio *folio, unsigned long addr,
> writable = !!pte_write(pte);
> if (any_young)
> young = !!pte_young(pte);
> + if (any_dirty)
> + dirty = !!pte_dirty(pte);
> pte = __pte_batch_clear_ignored(pte, flags);
>
> if (!pte_same(pte, expected_pte))
> @@ -193,6 +199,8 @@ static inline int folio_pte_batch(struct folio *folio, unsigned long addr,
> *any_writable |= writable;
> if (any_young)
> *any_young |= young;
> + if (any_dirty)
> + *any_dirty |= dirty;
>
> 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 f5e3699e7b54..d6f1889d6308 100644
> --- a/mm/madvise.c
> +++ b/mm/madvise.c
> @@ -321,6 +321,39 @@ static inline bool can_do_file_pageout(struct vm_area_struct *vma)
> file_permission(vma->vm_file, MAY_WRITE) == 0;
> }
>
> +static inline int madvise_folio_pte_batch(unsigned long addr, unsigned long end,
> + struct folio *folio, pte_t *ptep,
> + pte_t pte, bool *any_young,
> + bool *any_dirty)
> +{
> + int max_nr = (end - addr) / PAGE_SIZE;
> + const fpb_t fpb_flags = FPB_IGNORE_DIRTY | FPB_IGNORE_SOFT_DIRTY;
> +
> + return folio_pte_batch(folio, addr, ptep, pte, max_nr, fpb_flags, NULL,
> + any_young, any_dirty);
> +}
> +
> +static inline bool madvise_pte_split_folio(struct mm_struct *mm, pmd_t *pmd,
> + unsigned long addr,
> + struct folio *folio, pte_t **pte,
> + spinlock_t **ptl)
> +{
> + int err;
> +
> + if (!folio_trylock(folio))
> + return false;
> +
> + folio_get(folio);
> + pte_unmap_unlock(*pte, *ptl);
> + err = split_folio(folio);
> + folio_unlock(folio);
> + folio_put(folio);
> +
> + *pte = pte_offset_map_lock(mm, pmd, addr, ptl);
> +
> + return err == 0;
> +}
> +
> static int madvise_cold_or_pageout_pte_range(pmd_t *pmd,
> unsigned long addr, unsigned long end,
> struct mm_walk *walk)
> @@ -456,41 +489,30 @@ static int madvise_cold_or_pageout_pte_range(pmd_t *pmd,
> * next pte in the range.
> */
> if (folio_test_large(folio)) {
> - 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);
> + nr = madvise_folio_pte_batch(addr, end, folio, pte,
> + ptent, &any_young, NULL);
>
> 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 (madvise_pte_split_folio(mm, pmd, addr,
> + folio, &start_pte, &ptl))
> + nr = 0;
> if (!start_pte)
> break;
> + pte = start_pte;
> arch_enter_lazy_mmu_mode();
> - if (!err)
> - nr = 0;
> continue;
> }
> +
> + if (any_young)
> + ptent = pte_mkyoung(ptent);
> }
>
> /*
> @@ -688,44 +710,48 @@ static int madvise_free_pte_range(pmd_t *pmd, unsigned long addr,
> continue;
>
> /*
> - * If pmd isn't transhuge but the folio is large and
> - * is owned by only this process, split it and
> - * deactivate all pages.
> + * 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 marked as lazyfree. 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;
> + bool any_young, any_dirty;
>
> - if (folio_likely_mapped_shared(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;
> + nr = madvise_folio_pte_batch(addr, end, folio, pte,
> + ptent, &any_young, &any_dirty);
> +
> + if (nr < folio_nr_pages(folio)) {
> + if (folio_likely_mapped_shared(folio))
> + continue;
> +
> + arch_leave_lazy_mmu_mode();
> + if (madvise_pte_split_folio(mm, pmd, addr,
> + folio, &start_pte, &ptl))
> + nr = 0;
> + if (!start_pte)
> + break;
> + pte = start_pte;
> + arch_enter_lazy_mmu_mode();
> + continue;
> + }
> +
> + if (any_young)
> + ptent = pte_mkyoung(ptent);
> + if (any_dirty)
> + ptent = pte_mkdirty(ptent);
> }
>
> if (folio_test_swapcache(folio) || folio_test_dirty(folio)) {
> if (!folio_trylock(folio))
> continue;
> /*
> - * If folio is shared with others, we mustn't clear
> - * the folio's dirty flag.
> + * 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_mapcount(folio) != 1) {
> + if (folio_mapcount(folio) != folio_nr_pages(folio)) {
> folio_unlock(folio);
> continue;
> }
> @@ -741,19 +767,10 @@ static int madvise_free_pte_range(pmd_t *pmd, unsigned long addr,
> }
>
> if (pte_young(ptent) || pte_dirty(ptent)) {
> - /*
> - * Some of architecture(ex, PPC) don't update TLB
> - * with set_pte_at and tlb_remove_tlb_entry so for
> - * the portability, remap the pte with old|clean
> - * after pte clearing.
> - */
> - ptent = ptep_get_and_clear_full(mm, addr, pte,
> - tlb->fullmm);
> -
> - ptent = pte_mkold(ptent);
> - ptent = pte_mkclean(ptent);
> - set_pte_at(mm, addr, pte, ptent);
> - tlb_remove_tlb_entry(tlb, pte, addr);
> + clear_young_dirty_ptes(vma, addr, pte, nr,
> + CYDP_CLEAR_YOUNG |
> + CYDP_CLEAR_DIRTY);
> + tlb_remove_tlb_entries(tlb, pte, nr, addr);
> }
> folio_mark_lazyfree(folio);
> }
> diff --git a/mm/memory.c b/mm/memory.c
> index 33d87b64d15d..9e07d1b9020c 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, NULL);
> + &any_writable, NULL, NULL);
> folio_ref_add(folio, nr);
> if (folio_test_anon(folio)) {
> if (unlikely(folio_try_dup_anon_rmap_ptes(folio, page,
> @@ -1558,7 +1558,7 @@ 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, NULL, NULL);
>
> zap_present_folio_ptes(tlb, vma, folio, page, pte, ptent, nr,
> addr, details, rss, force_flush,

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In reply to: Lance Yang: "[PATCH v8 3/3] mm/madvise: optimize lazyfreeing with mTHP in madvise_free"
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