Messages in this thread | | | Date | Thu, 18 Aug 2022 13:45:46 +0800 | Subject | Re: [PATCH] mm/damon: Validate if the pmd entry is present before accessing | From | Baolin Wang <> |
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On 8/18/2022 1:12 PM, Muchun Song wrote: > > >> On Aug 18, 2022, at 13:07, Baolin Wang <baolin.wang@linux.alibaba.com> wrote: >> >> >> >> On 8/18/2022 11:39 AM, Muchun Song wrote: >>>> On Aug 18, 2022, at 10:57, Baolin Wang <baolin.wang@linux.alibaba.com> wrote: >>>> >>>> >>>> >>>> 在 8/18/2022 10:41 AM, Muchun Song 写道: >>>>>> On Aug 17, 2022, at 14:21, Baolin Wang <baolin.wang@linux.alibaba.com> wrote: >>>>>> >>>>>> The pmd_huge() is used to validate if the pmd entry is mapped by a huge >>>>>> page, also including the case of non-present (migration or hwpoisoned) >>>>>> pmd entry on arm64 or x86 architectures. Thus we should validate if it >>>>>> is present before making the pmd entry old or getting young state, >>>>>> otherwise we can not get the correct corresponding page. >>>>>> >>>>>> Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com> >>>>>> --- >>>>>> mm/damon/vaddr.c | 10 ++++++++++ >>>>>> 1 file changed, 10 insertions(+) >>>>>> >>>>>> diff --git a/mm/damon/vaddr.c b/mm/damon/vaddr.c >>>>>> index 3c7b9d6..1d16c6c 100644 >>>>>> --- a/mm/damon/vaddr.c >>>>>> +++ b/mm/damon/vaddr.c >>>>>> @@ -304,6 +304,11 @@ static int damon_mkold_pmd_entry(pmd_t *pmd, unsigned long addr, >>>>>> >>>>>> if (pmd_huge(*pmd)) { >>>>>> ptl = pmd_lock(walk->mm, pmd); >>>>>> + if (!pmd_present(*pmd)) { >>>>> Unluckily, we should use pte_present here. See commit c9d398fa23788. We can use >>>>> huge_ptep_get() to get a hugetlb pte, so it’s better to put the check after >>>>> pmd_huge. >>>> >>>> IMO this is not the case for hugetlb, and the hugetlb case will be handled by damon_mkold_hugetlb_entry(), which already used pte_present() for hugetlb case. >>> Well, I thought it is hugetlb related since I saw the usage of pmd_huge. If it is THP case, why >>> not use pmd_trans_huge? >> >> IIUC, it can not guarantee the pmd is present if pmd_trans_huge() returns true on all architectures, at least on X86, we still need pmd_present() validation. So changing to pmd_trans_huge() does not make code simpler from my side, and I prefer to keep this patch. > > I am not suggesting you change it to pmd_trans_huge() in this patch, I am just expressing > my curious. At least, it is a little confusing to me.
OK.
>> >> Maybe we can send another cleanup patch to replace pmd_huge() with pmd_trans_huge() for THP case to make code more readable? How do you think? Thanks. > > Yep, make sense to me.
OK. I can add a cleanup patch in next version. Thanks for your input.
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