Messages in this thread | | | Date | Wed, 27 Mar 2024 10:28:52 +0100 | Subject | Re: [RFC PATCH v1 1/4] mm: Introduce ptep_get_lockless_norecency() | From | David Hildenbrand <> |
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On 26.03.24 17:39, Ryan Roberts wrote: > On 26/03/2024 16:27, David Hildenbrand wrote: >> On 15.02.24 13:17, Ryan Roberts wrote: >>> With the introduction of contpte mapping support for arm64, that >>> architecture's implementation of ptep_get_lockless() has become very >>> complex due to the need to gather access and dirty bits from across all >>> of the ptes in the contpte block. This requires careful implementation >>> to ensure the returned value is consistent (because its not possible to >>> read all ptes atomically), but even in the common case when there is no >>> racing modification, we have to read all ptes, which gives an ~O(n^2) >>> cost if the core-mm is iterating over a range, and performing a >>> ptep_get_lockless() on each pte. >>> >>> Solve this by introducing ptep_get_lockless_norecency(), which does not >>> make any guarantees about access and dirty bits. Therefore it can simply >>> read the single target pte. >>> >>> At the same time, convert all call sites that previously used >>> ptep_get_lockless() but don't care about access and dirty state. >>> >> >> I'd probably split that part off. > > I thought the general guidance was to introduce new APIs in same patch they are > first used in? If I split this off, I'll have one patch for a new (unused) API, > then another for the first users.
I don't know what exact guidance there is, but I tend to leave "non trivial changes" to separate patches.
Some of the changes here are rather trivial (mm/hugetlb.c), and I agree that we can perform them here.
At least the "vmf.orig_pte" looked "non-trivial" to me, thus my comment.
> >> >>> We may want to do something similar for ptep_get() (i.e. >>> ptep_get_norecency()) in future; it doesn't suffer from the consistency >>> problem because the PTL serializes it with any modifications, but does >>> suffer the same O(n^2) cost. >>> >>> Signed-off-by: Ryan Roberts <ryan.roberts@arm.com> >>> --- >>> include/linux/pgtable.h | 37 ++++++++++++++++++++++++++++++++++--- >>> kernel/events/core.c | 2 +- >>> mm/hugetlb.c | 2 +- >>> mm/khugepaged.c | 2 +- >>> mm/memory.c | 2 +- >>> mm/swap_state.c | 2 +- >>> mm/swapfile.c | 2 +- >>> 7 files changed, 40 insertions(+), 9 deletions(-) >>> >>> diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h >>> index a36cf4e124b0..9dd40fdbd825 100644 >>> --- a/include/linux/pgtable.h >>> +++ b/include/linux/pgtable.h >>> @@ -528,16 +528,47 @@ static inline pmd_t pmdp_get_lockless(pmd_t *pmdp) >>> #endif /* CONFIG_PGTABLE_LEVELS > 2 */ >>> #endif /* CONFIG_GUP_GET_PXX_LOW_HIGH */ >>> >>> -/* >>> - * We require that the PTE can be read atomically. >>> - */ >>> #ifndef ptep_get_lockless >>> +/** >>> + * ptep_get_lockless - Get a pte without holding the page table lock. Young and >>> + * dirty bits are guaranteed to accurately reflect the state >>> + * of the pte at the time of the call. >>> + * @ptep: Page table pointer for pte to get. >>> + * >>> + * If young and dirty information is not required, use >>> + * ptep_get_lockless_norecency() which can be faster on some architectures. >>> + * >>> + * May be overridden by the architecture; otherwise, implemented using >>> + * ptep_get(), on the assumption that it is atomic. >>> + * >>> + * Context: Any. >>> + */ >> >> I think we usually say "Any context.". But I would just do it like idr.h: >> >> "Any context. It is safe to call this function without locking in your code." >> >> ... but is this true? We really want to say "without page table lock". Because >> there must be some way to prevent against concurrent page table freeing. For >> example, GUP-fast disables IRQs, whereby page table freeing code frees using RCU. > > How about: > > " > Context: Any context that guarrantees the page table can't be freed
s/guarrantees/guarantees/
> concurrently. The page table lock is not required. > " >
Sounds good.
>> >>> static inline pte_t ptep_get_lockless(pte_t *ptep) >>> { >>> return ptep_get(ptep); >>> } >>> #endif >>> >>> +#ifndef ptep_get_lockless_norecency >>> +/** >>> + * ptep_get_lockless_norecency - Get a pte without holding the page table lock. >>> + * Young and dirty bits may not be accurate. >>> + * @ptep: Page table pointer for pte to get. >>> + * >>> + * Prefer this over ptep_get_lockless() when young and dirty information is not >>> + * required since it can be faster on some architectures. >>> + * >>> + * May be overridden by the architecture; otherwise, implemented using the more >>> + * precise ptep_get_lockless(). >>> + * >>> + * Context: Any. >> >> Same comment. >> >>> + */ >>> +static inline pte_t ptep_get_lockless_norecency(pte_t *ptep) >>> +{ >>> + return ptep_get_lockless(ptep); >>> +} >>> +#endif >> >> [...] >> >>> diff --git a/mm/hugetlb.c b/mm/hugetlb.c >>> index 68283e54c899..41dc44eb8454 100644 >>> --- a/mm/hugetlb.c >>> +++ b/mm/hugetlb.c >>> @@ -7517,7 +7517,7 @@ pte_t *huge_pte_alloc(struct mm_struct *mm, struct >>> vm_area_struct *vma, >>> } >>> >>> if (pte) { >>> - pte_t pteval = ptep_get_lockless(pte); >>> + pte_t pteval = ptep_get_lockless_norecency(pte); >>> >>> BUG_ON(pte_present(pteval) && !pte_huge(pteval)); >>> } >>> diff --git a/mm/khugepaged.c b/mm/khugepaged.c >>> index 2771fc043b3b..1a6c9ed8237a 100644 >>> --- a/mm/khugepaged.c >>> +++ b/mm/khugepaged.c >>> @@ -1019,7 +1019,7 @@ static int __collapse_huge_page_swapin(struct mm_struct >>> *mm, >>> } >>> } >>> >>> - vmf.orig_pte = ptep_get_lockless(pte); >>> + vmf.orig_pte = ptep_get_lockless_norecency(pte); >>> if (!is_swap_pte(vmf.orig_pte)) >>> continue; >> >> >> Hm, I think you mentioned that we want to be careful with vmf.orig_pte. > > Yeah good point. So I guess this should move to patch 3 (which may be dropped - > tbd)? >
Yes. Or a separate one where you explain in detail why do_swap_page() can handle it just fine.
-- Cheers,
David / dhildenb
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