Re: [PATCH] mm/damon: Validate if the pmd entry is present before accessing

[Baolin Wang <baolin.wang@linux.alibaba.com>]

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.