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Subject: Re: [PATCH v2] mm: hwpoison: disable memory error handling on 1GB
 hugepage
To:     Wanpeng Li <kernellwp@gmail.com>
Cc:     Michael Ellerman <mpe@ellerman.id.au>,
        Andrew Morton <akpm@linux-foundation.org>,
        Punit Agrawal <punit.agrawal@arm.com>,
        Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>,
        "linux-mm@kvack.org" <linux-mm@kvack.org>,
        Michal Hocko <mhocko@kernel.org>,
        "Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com>,
        Anshuman Khandual <khandual@linux.vnet.ibm.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        "linuxppc-dev@lists.ozlabs.org" <linuxppc-dev@lists.ozlabs.org>,
        kvm <kvm@vger.kernel.org>, Paolo Bonzini <pbonzini@redhat.com>,
        Xiao Guangrong <xiaoguangrong@tencent.com>,
        lidongchen@tencent.com, yongkaiwu@tencent.com
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From:   Mike Kravetz <mike.kravetz@oracle.com>
Message-ID: <f79d828c-b0b4-8a20-c316-a13430cfb13c@oracle.com>
Date:   Wed, 29 May 2019 16:31:01 -0700
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On 5/28/19 2:49 AM, Wanpeng Li wrote:
> Cc Paolo,
> Hi all,
> On Wed, 14 Feb 2018 at 06:34, Mike Kravetz <mike.kravetz@oracle.com> wrote:
>>
>> On 02/12/2018 06:48 PM, Michael Ellerman wrote:
>>> Andrew Morton <akpm@linux-foundation.org> writes:
>>>
>>>> On Thu, 08 Feb 2018 12:30:45 +0000 Punit Agrawal <punit.agrawal@arm.com> wrote:
>>>>
>>>>>>
>>>>>> So I don't think that the above test result means that errors are properly
>>>>>> handled, and the proposed patch should help for arm64.
>>>>>
>>>>> Although, the deviation of pud_huge() avoids a kernel crash the code
>>>>> would be easier to maintain and reason about if arm64 helpers are
>>>>> consistent with expectations by core code.
>>>>>
>>>>> I'll look to update the arm64 helpers once this patch gets merged. But
>>>>> it would be helpful if there was a clear expression of semantics for
>>>>> pud_huge() for various cases. Is there any version that can be used as
>>>>> reference?
>>>>
>>>> Is that an ack or tested-by?
>>>>
>>>> Mike keeps plaintively asking the powerpc developers to take a look,
>>>> but they remain steadfastly in hiding.
>>>
>>> Cc'ing linuxppc-dev is always a good idea :)
>>>
>>
>> Thanks Michael,
>>
>> I was mostly concerned about use cases for soft/hard offline of huge pages
>> larger than PMD_SIZE on powerpc.  I know that powerpc supports PGD_SIZE
>> huge pages, and soft/hard offline support was specifically added for this.
>> See, 94310cbcaa3c "mm/madvise: enable (soft|hard) offline of HugeTLB pages
>> at PGD level"
>>
>> This patch will disable that functionality.  So, at a minimum this is a
>> 'heads up'.  If there are actual use cases that depend on this, then more
>> work/discussions will need to happen.  From the e-mail thread on PGD_SIZE
>> support, I can not tell if there is a real use case or this is just a
>> 'nice to have'.
> 
> 1GB hugetlbfs pages are used by DPDK and VMs in cloud deployment, we
> encounter gup_pud_range() panic several times in product environment.
> Is there any plan to reenable and fix arch codes?

I too am aware of slightly more interest in 1G huge pages.  Suspect that as
Intel MMU capacity increases to handle more TLB entries there will be more
and more interest.

Personally, I am not looking at this issue.  Perhaps Naoya will comment as
he know most about this code.

> In addition, https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/arch/x86/kvm/mmu.c#n3213
> The memory in guest can be 1GB/2MB/4K, though the host-backed memory
> are 1GB hugetlbfs pages, after above PUD panic is fixed,
> try_to_unmap() which is called in MCA recovery path will mark the PUD
> hwpoison entry. The guest will vmexit and retry endlessly when
> accessing any memory in the guest which is backed by this 1GB poisoned
> hugetlbfs page. We have a plan to split this 1GB hugetblfs page by 2MB
> hugetlbfs pages/4KB pages, maybe file remap to a virtual address range
> which is 2MB/4KB page granularity, also split the KVM MMU 1GB SPTE
> into 2MB/4KB and mark the offensive SPTE w/ a hwpoison flag, a sigbus
> will be delivered to VM at page fault next time for the offensive
> SPTE. Is this proposal acceptable?

I am not sure of the error handling design, but this does sound reasonable.
That block of code which potentially dissolves a huge page on memory error
is hard to understand and I'm not sure if that is even the 'normal'
functionality.  Certainly, we would hate to waste/poison an entire 1G page
for an error on a small subsection.

-- 
Mike Kravetz