Re: [PATCH v5 2/6] mm: swap: free_swap_and_cache_nr() as batched free_swap_and_cache()

[Zi Yan <ziy@nvidia.com>]

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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);

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Best Regards,
Yan, Zi

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