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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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