linux-kernel - Re: [RFC PATCH v1 1/4] mm: Introduce ptep_get_lockless

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Date: Wed, 27 Mar 2024 18:02:29 +0100
From: David Hildenbrand <david@...hat.com>
To: Ryan Roberts <ryan.roberts@....com>, Mark Rutland <mark.rutland@....com>,
 Catalin Marinas <catalin.marinas@....com>, Will Deacon <will@...nel.org>,
 Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
 Jiri Olsa <jolsa@...nel.org>, Ian Rogers <irogers@...gle.com>,
 Adrian Hunter <adrian.hunter@...el.com>,
 Andrew Morton <akpm@...ux-foundation.org>,
 Muchun Song <muchun.song@...ux.dev>
Cc: linux-arm-kernel@...ts.infradead.org, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH v1 1/4] mm: Introduce ptep_get_lockless_norecency()

>>>>
>>>>> 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.
> 
> Ahh no wait - I remember now; the reason I believe this is a "trivial" case is
> because we only leak vmf.orig_pte to the rest of the world if its a swap entry.

Ugh, yes!

-- 
Cheers,

David / dhildenb