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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: <Y1piguJXagcxiTpn@arm.com>
Date:   Fri, 28 Oct 2022 10:45:09 +0800
Cc:     Anshuman Khandual <anshuman.khandual@arm.com>,
        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>,
        Linux Memory Management List <linux-mm@kvack.org>,
        linux-kernel@vger.kernel.org
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Message-Id: <ED740104-0A97-44C1-839F-407F221070BB@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> <Y1piguJXagcxiTpn@arm.com>
To:     Catalin Marinas <catalin.marinas@arm.com>
Precedence: bulk


> On Oct 27, 2022, at 18:50, Catalin Marinas <catalin.marinas@arm.com> =
wrote:
>=20
> On Wed, Oct 26, 2022 at 02:06:00PM +0530, Anshuman Khandual wrote:
>> On 10/26/22 12:31, Muchun Song wrote:
>>>> On 10/25/22 12:06, Muchun Song wrote:
>>>>>> On Oct 25, 2022, at 09:42, Wupeng Ma <mawupeng1@huawei.com> =
wrote:
>>>>>> 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 ?
>=20
> For the record, as we discussed offline, changing the output address
> (pfn) of a pte is not safe without break-before-make if at least one =
of
> the mappings was writeable. The caller (vmemmap_remap_pte()) would =
need
> to be fixed to first invalidate the pte and then write the new pte. I

Hi Catalin,

Could you expose more details about what issue it will be caused? I am
not familiar with arm64.

> assume no other CPU accesses this part of the vmemmap while the pte is
> being remapped.

However, there is no guarantee that no other CPU accesses this pte.
E.g. memory failure or memory compaction, both can obtain head page
from any tail struct pages (only read) anytime.

Thanks.

>=20
> --=20
> Catalin