Messages in this thread | ![/](/images/icornerl.gif) | | Date | Fri, 5 Apr 2024 11:24:36 +0200 | Subject | Re: [PATCH v5 2/6] mm: swap: free_swap_and_cache_nr() as batched free_swap_and_cache() | From | David Hildenbrand <> |
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On 03.04.24 09:21, Ryan Roberts wrote: > 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.
Yes please. I remember Willy found that variant to be most micro-optimized.
-- Cheers,
David / dhildenb
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