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Subject: Re: [PATCH -next 1/1] mm: hugetlb_vmemmap: Fix WARN_ON in
 vmemmap_remap_pte
From:   Muchun Song <muchun.song@linux.dev>
In-Reply-To: <3c545133-71aa-9a8d-8a13-09186c4fa767@arm.com>
Date:   Wed, 26 Oct 2022 16:53:33 +0800
Cc:     Wupeng Ma <mawupeng1@huawei.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Mike Kravetz <mike.kravetz@oracle.com>,
        Muchun Song <songmuchun@bytedance.com>,
        Michal Hocko <mhocko@suse.com>,
        Oscar Salvador <osalvador@suse.de>, catalin.marinas@arm.com,
        Linux Memory Management List <linux-mm@kvack.org>,
        linux-kernel@vger.kernel.org
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Message-Id: <46A852E1-B19E-4ABE-B96C-992DC7C67414@linux.dev>
References: <20221025014215.3466904-1-mawupeng1@huawei.com>
 <614E3E83-1EAB-4C39-AF9C-83C0CCF26218@linux.dev>
 <35dd51eb-c266-f221-298a-21309c17971a@arm.com>
 <3D6FDA43-A812-4907-B9C8-C2B25567DBBC@linux.dev>
 <3c545133-71aa-9a8d-8a13-09186c4fa767@arm.com>
To:     Anshuman Khandual <anshuman.khandual@arm.com>
Precedence: bulk


> On Oct 26, 2022, at 16:36, Anshuman Khandual =
<anshuman.khandual@arm.com> wrote:
>=20
>=20
>=20
> On 10/26/22 12:31, Muchun Song wrote:
>>=20
>>=20
>>> On Oct 26, 2022, at 13:06, Anshuman Khandual =
<anshuman.khandual@arm.com> wrote:
>>>=20
>>>=20
>>>=20
>>> On 10/25/22 12:06, Muchun Song wrote:
>>>>=20
>>>>=20
>>>>> On Oct 25, 2022, at 09:42, Wupeng Ma <mawupeng1@huawei.com> wrote:
>>>>>=20
>>>>> From: Ma Wupeng <mawupeng1@huawei.com>
>>>>>=20
>>>>> Commit f41f2ed43ca5 ("mm: hugetlb: free the vmemmap pages =
associated with
>>>>> each HugeTLB page") add vmemmap_remap_pte to remap the tail pages =
as
>>>>> read-only to catch illegal write operation to the tail page.
>>>>>=20
>>>>> However this will lead to WARN_ON in arm64 in =
__check_racy_pte_update()
>>>>=20
>>>> Thanks for your finding this issue.
>>>>=20
>>>>> since this may lead to dirty state cleaned. This check is =
introduced by
>>>>> commit 2f4b829c625e ("arm64: Add support for hardware updates of =
the
>>>>> access and dirty pte bits") and the initial check is as follow:
>>>>>=20
>>>>> BUG_ON(pte_write(*ptep) && !pte_dirty(pte));
>>>>>=20
>>>>> Since we do need to mark this pte as read-only to catch illegal =
write
>>>>> operation to the tail pages, use set_pte  to replace set_pte_at to =
bypass
>>>>> this check.
>>>>=20
>>>> In theory, the waring does not affect anything since the tail =
vmemmap
>>>> pages are supposed to be read-only. So, skipping this check for =
vmemmap
>>>=20
>>> Tails vmemmap pages are supposed to be read-only, in practice but =
their
>>> backing pages do have pte_write() enabled. Otherwise the =
VM_WARN_ONCE()
>>> warning would not have triggered.
>>=20
>> Right.
>>=20
>>>=20
>>>       VM_WARN_ONCE(pte_write(old_pte) && !pte_dirty(pte),
>>>                    "%s: racy dirty state clearing: 0x%016llx -> =
0x%016llx",
>>>                    __func__, pte_val(old_pte), pte_val(pte));
>>>=20
>>> Also, is not it true that the pte being remapped into a different =
page
>>> as read only, than what it had originally (which will be freed up) =
i.e=20
>>> the PFN in 'old_pte' and 'pte' will be different. Hence is there =
still
>>=20
>> Right.
>>=20
>>> a possibility for a race condition even when the PFN changes ?
>>=20
>> Sorry, I didn't get this question. Did you mean the PTE is changed =
from
>> new (pte) to the old one (old_pte) by the hardware because of the =
update
>> of dirty bit when a concurrent write operation to the tail vmemmap =
page?
>=20
> No, but is not vmemmap_remap_pte() reuses walk->reuse_page for all =
remaining
> tails pages ? Is not there a PFN change, along with access permission =
change
> involved in this remapping process ?

Alright, yes, both the PFN and the access permission are changed.