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    SubjectRe: [PATCH v9 4/4] mm/madvise: optimize lazyfreeing with mTHP in madvise_free
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    On 18.04.24 12:57, 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@arm.com
    > [2] https://lore.kernel.org/linux-mm/20240214204435.167852-1-david@redhat.com
    >
    > Reviewed-by: Ryan Roberts <ryan.roberts@arm.com>
    > Signed-off-by: Lance Yang <ioworker0@gmail.com>
    > ---
    > mm/madvise.c | 85 +++++++++++++++++++++++++++-------------------------
    > 1 file changed, 44 insertions(+), 41 deletions(-)
    >
    > diff --git a/mm/madvise.c b/mm/madvise.c
    > index 4597a3568e7e..375ab3234603 100644
    > --- a/mm/madvise.c
    > +++ b/mm/madvise.c
    > @@ -643,6 +643,7 @@ static int madvise_free_pte_range(pmd_t *pmd, unsigned long addr,
    > unsigned long end, struct mm_walk *walk)
    >
    > {
    > + const cydp_t cydp_flags = CYDP_CLEAR_YOUNG | CYDP_CLEAR_DIRTY;
    > struct mmu_gather *tlb = walk->private;
    > struct mm_struct *mm = tlb->mm;
    > struct vm_area_struct *vma = walk->vma;
    > @@ -697,44 +698,57 @@ 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, NULL);
    > +
    > + if (nr < folio_nr_pages(folio)) {
    > + int err;
    > +
    > + if (folio_likely_mapped_shared(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);

    I'd just put it on a single line.

    > + if (!start_pte)
    > + break;
    > + arch_enter_lazy_mmu_mode();
    > + if (!err)
    > + nr = 0;
    > + continue;
    > + }
    > +
    > + if (any_young)
    > + ptent = pte_mkyoung(ptent);
    > + if (any_dirty)

    any_dirty is never set, likely missed to pass it to
    madvise_folio_pte_batch().

    Apart from that LGTM and this patch is much easier to review now!


    With above:

    Acked-by: David Hildenbrand <david@redhat.com>

    --
    Cheers,

    David / dhildenb


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