[RFC PATCH v1 1/4] mm: Introduce ptep_get_lockless_norecency()

[David Hildenbrand]

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 at 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