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

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Date:   Thu, 18 Aug 2022 13:12:16 +0800
From:   Muchun Song <muchun.song@...ux.dev>
To:     Baolin Wang <baolin.wang@...ux.alibaba.com>
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



> On Aug 18, 2022, at 13:07, Baolin Wang <baolin.wang@...ux.alibaba.com> wrote:
> 
> 
> 
> On 8/18/2022 11:39 AM, Muchun Song wrote:
>>> On Aug 18, 2022, at 10:57, Baolin Wang <baolin.wang@...ux.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@...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.
>> 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.


> 
> 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.

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