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Message-ID: <CAK1f24kh-vN3_6ZLUZWXu_g1UOPBuoVXq2jsnx_GBbGFXyXRFQ@mail.gmail.com>
Date: Thu, 11 Apr 2024 20:46:43 +0800
From: Lance Yang <ioworker0@...il.com>
To: Ryan Roberts <ryan.roberts@....com>
Cc: akpm@...ux-foundation.org, david@...hat.com, 21cnbao@...il.com,
mhocko@...e.com, fengwei.yin@...el.com, zokeefe@...gle.com,
shy828301@...il.com, xiehuan09@...il.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 v5 1/2] mm/madvise: optimize lazyfreeing with mTHP in madvise_free
On Thu, Apr 11, 2024 at 7:11 PM Ryan Roberts <ryan.roberts@....com> wrote:
>
> 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@armcom
> > [2] https://lore.kernel.org/linux-mm/20240214204435.167852-1-david@redhat.com
> >
> > Signed-off-by: Lance Yang <ioworker0@...il.com>
> > ---
> > 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.
Sorry for that. I did see this warning msg, but I didn't take it seriously :(
>
> > - 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.
I'm not sure if it's safe to call folio_mapcount() without holding the
folio lock.
As mentioned earlier by David in the v2[1]
> What could work for large folios is making sure that #ptes that map the
> folio here correspond to the folio_mapcount(). And folio_mapcount()
> should be called under folio lock, to avoid racing with swapout/migration.
[1] https://lore.kernel.org/all/5cc05529-eb80-410e-bc26-233b0ba0b21f@redhatcom/
Thanks,
Lance
>
> 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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