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

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Date:   Thu, 18 Aug 2022 10:57:21 +0800
From:   Baolin Wang <baolin.wang@...ux.alibaba.com>
To:     Muchun Song <muchun.song@...ux.dev>
Cc:     sj@...nel.org, Andrew Morton <akpm@...ux-foundation.org>,
        damon@...ts.linux.dev, Linux MM <linux-mm@...ck.org>,
        linux-kernel@...r.kernel.org,
        Mike Kravetz <mike.kravetz@...cle.com>
Subject: Re: [PATCH] mm/damon: Validate if the pmd entry is present before
 accessing



在 8/18/2022 10:41 AM, Muchun Song 写道:
> 
> 
>> On Aug 17, 2022, at 14:21, Baolin Wang <baolin.wang@...ux.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@...ux.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.

> 
> 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
>>
>>
>