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

[Lance Yang]

Hey David,

Thanks a lot for taking time to review!

On Thu, Apr 18, 2024 at 1:05 AM David Hildenbrand <david@xxxxxxxxxx> 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@xxxxxxxxxx
> >
> > Signed-off-by: Lance Yang <ioworker0@xxxxxxxxx>
>
> 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
>