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

[Muchun Song <muchun.song@linux.dev>]

> 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?

Thanks.

> 
>> Cc Mike to make sure I am not missing something.
>> Muchun,
>> Thanks.
>>> +			spin_unlock(ptl);
>>> +			return 0;
>>> +		}
>>> +
>>> 		if (pmd_huge(*pmd)) {
>>> 			damon_pmdp_mkold(pmd, walk->mm, addr);
>>> 			spin_unlock(ptl);
>>> @@ -431,6 +436,11 @@ static int damon_young_pmd_entry(pmd_t *pmd, unsigned long addr,
>>> #ifdef CONFIG_TRANSPARENT_HUGEPAGE
>>> 	if (pmd_huge(*pmd)) {
>>> 		ptl = pmd_lock(walk->mm, pmd);
>>> +		if (!pmd_present(*pmd)) {
>>> +			spin_unlock(ptl);
>>> +			return 0;
>>> +		}
>>> +
>>> 		if (!pmd_huge(*pmd)) {
>>> 			spin_unlock(ptl);
>>> 			goto regular_page;
>>> -- 
>>> 1.8.3.1