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SubjectRe: [PATCH v5 2/6] mm: swap: free_swap_and_cache_nr() as batched free_swap_and_cache()
April 3, 2024 at 8:30 AM, "Zi Yan" <ziy@nvidia.com> wrote:



>
> On 27 Mar 2024, at 10:45, Ryan Roberts wrote:
>
> >
> > Now that we no longer have a convenient flag in the cluster to determine
> >
> > if a folio is large, free_swap_and_cache() will take a reference and
> >
> > lock a large folio much more often, which could lead to contention and
> >
> > (e.g.) failure to split large folios, etc.
> >
> > Let's solve that problem by batch freeing swap and cache with a new
> >
> > function, free_swap_and_cache_nr(), to free a contiguous range of swap
> >
> > entries together. This allows us to first drop a reference to each swap
> >
> > slot before we try to release the cache folio. This means we only try to
> >
> > release the folio once, only taking the reference and lock once - much
> >
> > better than the previous 512 times for the 2M THP case.
> >
> > Contiguous swap entries are gathered in zap_pte_range() and
> >
> > madvise_free_pte_range() in a similar way to how present ptes are
> >
> > already gathered in zap_pte_range().
> >
> > While we are at it, let's simplify by converting the return type of both
> >
> > functions to void. The return value was used only by zap_pte_range() to
> >
> > print a bad pte, and was ignored by everyone else, so the extra
> >
> > reporting wasn't exactly guaranteed. We will still get the warning with
> >
> > most of the information from get_swap_device(). With the batch version,
> >
> > we wouldn't know which pte was bad anyway so could print the wrong one.
> >
> > Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>
> >
> > ---
> >
> > include/linux/pgtable.h | 28 +++++++++++++++
> >
> > include/linux/swap.h | 12 +++++--
> >
> > mm/internal.h | 48 +++++++++++++++++++++++++
> >
> > mm/madvise.c | 12 ++++---
> >
> > mm/memory.c | 13 +++----
> >
> > mm/swapfile.c | 78 ++++++++++++++++++++++++++++++-----------
> >
> > 6 files changed, 157 insertions(+), 34 deletions(-)
> >
> > diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
> >
> > index 09c85c7bf9c2..8185939df1e8 100644
> >
> > --- a/include/linux/pgtable.h
> >
> > +++ b/include/linux/pgtable.h
> >
> > @@ -708,6 +708,34 @@ static inline void pte_clear_not_present_full(struct mm_struct *mm,
> >
> > }
> >
> > #endif
> >
> > +#ifndef clear_not_present_full_ptes
> >
> > +/**
> >
> > + * clear_not_present_full_ptes - Clear consecutive not present PTEs.
> >
> > + * @mm: Address space the ptes represent.
> >
> > + * @addr: Address of the first pte.
> >
> > + * @ptep: Page table pointer for the first entry.
> >
> > + * @nr: Number of entries to clear.
> >
> > + * @full: Whether we are clearing a full mm.
> >
> > + *
> >
> > + * May be overridden by the architecture; otherwise, implemented as a simple
> >
> > + * loop over pte_clear_not_present_full().
> >
> > + *
> >
> > + * Context: The caller holds the page table lock. The PTEs are all not present.
> >
> > + * The PTEs are all in the same PMD.
> >
> > + */
> >
> > +static inline void clear_not_present_full_ptes(struct mm_struct *mm,
> >
> > + unsigned long addr, pte_t *ptep, unsigned int nr, int full)
> >
> > +{
> >
> > + for (;;) {
> >
> > + pte_clear_not_present_full(mm, addr, ptep, full);
> >
> > + if (--nr == 0)
> >
> > + break;
> >
> > + ptep++;
> >
> > + addr += PAGE_SIZE;
> >
> > + }
> >
> > +}
> >
> > +#endif
> >
> > +
> >
>
> Would the code below be better?
>
> for (i = 0; i < nr; i++, ptep++, addr += PAGE_SIZE)
>

FWIW

for (; nr-- > 0; ptep++, addr += PAGE_SIZE)
pte_clear_not_present_full(mm, addr, ptep, full);

Thanks,
Lance


> pte_clear_not_present_full(mm, addr, ptep, full);
>
> --
>
> Best Regards,
>
> Yan, Zi
>

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