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Message-ID: <CAK1f24=6weNdHMKf=Q9JwbcShNG0b7bFFh1wFH=q2wDTWKd4Yw@mail.gmail.com>
Date: Thu, 18 Apr 2024 08:18:23 +0800
From: Lance Yang <ioworker0@...il.com>
To: David Hildenbrand <david@...hat.com>
Cc: akpm@...ux-foundation.org, ryan.roberts@....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 v8 3/3] mm/madvise: optimize lazyfreeing with mTHP in madvise_free
Hey David,
Thanks a lot for taking time to review!
On Thu, Apr 18, 2024 at 1:05 AM David Hildenbrand <david@...hat.com> wrote:
>
> On 17.04.24 16:14, 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@armcom
> > [2] https://lore.kernel.org/linux-mm/20240214204435.167852-1-david@redhat.com
> >
> > Signed-off-by: Lance Yang <ioworker0@...il.com>
>
> Some of the changes could have been moved into separate patches to ease
> review ;)
>
> At least the folio_pte_batch() change and factoring out some stuff from
> madvise_cold_or_pageout_pte_range(). But see below on the latter.
Thanks for your suggestion! It makes sense to split some of the changes
into separate patches for easier review :)
>
> > ---
> > mm/internal.h | 12 ++++-
> > mm/madvise.c | 141 ++++++++++++++++++++++++++++----------------------
> > mm/memory.c | 4 +-
> > 3 files changed, 91 insertions(+), 66 deletions(-)
>
> [...]
>
> > 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;
>
> Reverse Christmas tree looks nicer ;)
Yep, I understand.
>
> > +
> > + 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);
>
> Staring at this helper again, I am really not sure if we should have it.
> Calling semantics are "special" and that pte_t **pte is just ...
> "special" as well ;)
>
> Can we just leave that part as is, in the caller? That would also mean
> less madvise_cold_or_pageout_pte_range() churn ... which i would welcome
> as part of this patch.
Yep, let's leave that part as it is in the caller :)
>
> [...]
>
> > @@ -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);
>
> That indent looks odd. I suggest simply having a local variable
>
> const cydp_t cydp_flags = CYDP_CLEAR_YOUNG | CYDP_CLEAR_DIRTY;
>
> and then use cydp_flags here that will make this easier to read.
Nice. I'll use cydp_flags here in the next version.
Thanks again for the review!
Lance
>
> --
> Cheers,
>
> David / dhildenb
>
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