Re: [PATCH v5 1/2] mm/madvise: optimize lazyfreeing with mTHP in madvise_free

From: Ryan Roberts
Date: Thu Apr 11 2024 - 07:12:09 EST

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On 08/04/2024 05:24, 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>
> ---
> include/linux/pgtable.h | 34 +++++++++
> mm/internal.h | 12 +++-
> mm/madvise.c | 149 ++++++++++++++++++++++------------------
> mm/memory.c | 4 +-
> 4 files changed, 129 insertions(+), 70 deletions(-)
>
> diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
> index 0f4b2faa1d71..4dd442787420 100644
> --- a/include/linux/pgtable.h
> +++ b/include/linux/pgtable.h
> @@ -489,6 +489,40 @@ static inline pte_t ptep_get_and_clear(struct mm_struct *mm,
> }
> #endif
>
> +#ifndef mkold_clean_ptes
> +/**
> + * mkold_clean_ptes - Mark PTEs that map consecutive pages of the same folio
> + * as old and clean.
> + * @mm: Address space 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 and clean.
> + *
> + * May be overridden by the architecture; otherwise, implemented by
> + * get_and_clear/modify/set for each pte in the range.
> + *
> + * 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_clean_ptes(struct mm_struct *mm, unsigned long addr,
> + pte_t *ptep, unsigned int nr)

Just thinking out loud, I wonder if it would be cleaner to convert mkold_ptes()
(which I added as part of swap-out) to something like:

clear_young_dirty_ptes(struct mm_struct *mm, unsigned long addr,
pte_t *ptep, unsigned int nr,
bool clear_young, bool clear_dirty);

Then we can use the same function for both use cases and also have the ability
to only clear dirty in future if we ever need it. The other advantage is that we
only need to plumb a single function down the arm64 arch code. As it currently
stands, those 2 functions would be duplicating most of their code.

Generated code would still be the same since I'd expect the callsites to be
passing in constants for clear_young and clear_dirty.

> +{
> + pte_t pte;
> +
> + for (;;) {
> + pte = ptep_get_and_clear(mm, addr, ptep);
> + set_pte_at(mm, addr, ptep, pte_mkclean(pte_mkold(pte)));
> + if (--nr == 0)
> + break;
> + ptep++;
> + addr += PAGE_SIZE;
> + }
> +}
> +#endif
> +
> static inline void ptep_clear(struct mm_struct *mm, unsigned long addr,
> pte_t *ptep)
> {
> diff --git a/mm/internal.h b/mm/internal.h
> index 57c1055d5568..792a9baf0d14 100644
> --- a/mm/internal.h
> +++ b/mm/internal.h
> @@ -132,6 +132,8 @@ 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.
> @@ -147,18 +149,20 @@ 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);
> @@ -174,6 +178,8 @@ 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))
> @@ -191,6 +197,8 @@ 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 bf26cf2b7715..0777df2e3691 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 +489,29 @@ 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;
> -

nit: there should be a blank line between variable declarations and following
code. You have removed it here (and similar in free function). Did you run
checkpatch.pl? It would have caught these things.

> - 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);
> }
>
> /*
> @@ -687,47 +708,54 @@ 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;
> + nr = madvise_folio_pte_batch(addr, end, folio, pte,
> + ptent, &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);
> + 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_trylock(folio))
> + continue;

This is still wrong. This should all be protected by the "if
(folio_test_swapcache(folio) || folio_test_dirty(folio))" as it was previously
so that you only call folio_trylock() if that condition is true. You are
unconditionally locking here, then unlocking, then relocking below if the
condition is met. Just put everything inside the condition and lock once.

Thanks,
Ryan

> + /*
> + * 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) != folio_nr_pages(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;
> }
> + folio_unlock(folio);
>
> 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 (folio_mapcount(folio) != 1) {
> - folio_unlock(folio);
> - continue;
> - }
>
> if (folio_test_swapcache(folio) &&
> !folio_free_swap(folio)) {
> @@ -740,19 +768,8 @@ 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);
> + mkold_clean_ptes(mm, addr, pte, nr);
> + tlb_remove_tlb_entries(tlb, pte, nr, addr);
> }
> folio_mark_lazyfree(folio);
> }
> diff --git a/mm/memory.c b/mm/memory.c
> index 1723c8ddf9cb..fe9d4d64c627 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,
> @@ -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, NULL, NULL);
>
> zap_present_folio_ptes(tlb, vma, folio, page, pte, ptent, nr,
> addr, details, rss, force_flush,

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