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    SubjectRe: [PATCH v5 2/6] mm: swap: free_swap_and_cache_nr() as batched free_swap_and_cache()
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    On 03/04/2024 01:30, Zi Yan 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)
    > pte_clear_not_present_full(mm, addr, ptep, full);

    I certainly agree that this is cleaner and more standard. But I'm copying the
    pattern used by the other batch helpers. I believe this pattern was first done
    by Willy for set_ptes(), then continued by DavidH for wrprotect_ptes() and
    clear_full_ptes().

    I guess the benefit is that ptep and addr are only incremented if we are going
    around the loop again. I'd rather continue to be consistent with those other
    helpers.


    >
    > --
    > Best Regards,
    > Yan, Zi


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