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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: <ad3f36ca-5027-45a5-cd48-dc32bf968c9d@linux.alibaba.com>
Date:   Thu, 18 Aug 2022 13:12:16 +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: <8C1A5089-2830-407D-8FF3-2F0F92984645@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>
 <6F99AE03-3C74-4E5B-87E1-6F3232BEEBB5@linux.dev>
 <ad3f36ca-5027-45a5-cd48-dc32bf968c9d@linux.alibaba.com>
To:     Baolin Wang <baolin.wang@linux.alibaba.com>
Precedence: bulk


> On Aug 18, 2022, at 13:07, Baolin Wang <baolin.wang@linux.alibaba.com> =
wrote:
>=20
>=20
>=20
> 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:
>>>=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?
>=20
> 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.


>=20
> 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.

>=20
>>>=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