Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Content-Type: text/plain;
        charset=utf-8
MIME-Version: 1.0
Subject: Re: [PATCH] mm/damon: Validate if the pmd entry is present before
 accessing
From:   Muchun Song <muchun.song@linux.dev>
In-Reply-To: <2838b6737bc259cf575ff11fd1c4b7fdb340fa73.1660717122.git.baolin.wang@linux.alibaba.com>
Date:   Thu, 18 Aug 2022 10:41:10 +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>
Content-Transfer-Encoding: quoted-printable
Message-Id: <5173E308-2403-4667-9162-865CF7F11838@linux.dev>
References: <2838b6737bc259cf575ff11fd1c4b7fdb340fa73.1660717122.git.baolin.wang@linux.alibaba.com>
To:     Baolin Wang <baolin.wang@linux.alibaba.com>
Precedence: bulk


> 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.

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
>=20
>=20