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Subject: Re: [PATCH] mm/damon: Validate if the pmd entry is present before
 accessing
From:   Muchun Song <muchun.song@linux.dev>
In-Reply-To: <ceaa1e54-e79a-86c7-cf35-d4159088f0f3@linux.alibaba.com>
Date:   Thu, 18 Aug 2022 11:39:04 +0800
Cc:     sj@kernel.org, Andrew Morton <akpm@linux-foundation.org>,
        damon@lists.linux.dev, Linux MM <linux-mm@kvack.org>,
        linux-kernel@vger.kernel.org,
        Mike Kravetz <mike.kravetz@oracle.com>
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Message-Id: <6F99AE03-3C74-4E5B-87E1-6F3232BEEBB5@linux.dev>
References: <2838b6737bc259cf575ff11fd1c4b7fdb340fa73.1660717122.git.baolin.wang@linux.alibaba.com>
 <5173E308-2403-4667-9162-865CF7F11838@linux.dev>
 <ceaa1e54-e79a-86c7-cf35-d4159088f0f3@linux.alibaba.com>
To:     Baolin Wang <baolin.wang@linux.alibaba.com>
Precedence: bulk


> On Aug 18, 2022, at 10:57, Baolin Wang <baolin.wang@linux.alibaba.com> =
wrote:
>=20
>=20
>=20
> =E5=9C=A8 8/18/2022 10:41 AM, Muchun Song =E5=86=99=E9=81=93:
>>> On Aug 17, 2022, at 14:21, Baolin Wang =
<baolin.wang@linux.alibaba.com> wrote:
>>>=20
>>> 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.
>>>=20
>>> Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>
>>> ---
>>> mm/damon/vaddr.c | 10 ++++++++++
>>> 1 file changed, 10 insertions(+)
>>>=20
>>> 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,
>>>=20
>>> 	if (pmd_huge(*pmd)) {
>>> 		ptl =3D 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=E2=80=99s better to put =
the check after
>> pmd_huge.
>=20
> 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.

>=20
>> 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 =3D pmd_lock(walk->mm, pmd);
>>> +		if (!pmd_present(*pmd)) {
>>> +			spin_unlock(ptl);
>>> +			return 0;
>>> +		}
>>> +
>>> 		if (!pmd_huge(*pmd)) {
>>> 			spin_unlock(ptl);
>>> 			goto regular_page;
>>> --=20
>>> 1.8.3.1