Date:   Tue, 11 Jun 2019 16:42:39 +0800
From:   Wanpeng Li <kernellwp@...il.com>
To:     Naoya Horiguchi <n-horiguchi@...jp.nec.com>
Cc:     Mike Kravetz <mike.kravetz@...cle.com>,
        Michael Ellerman <mpe@...erman.id.au>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Punit Agrawal <punit.agrawal@....com>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        Michal Hocko <mhocko@...nel.org>,
        "Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>,
        Anshuman Khandual <khandual@...ux.vnet.ibm.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        "linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>,
        kvm <kvm@...r.kernel.org>, Paolo Bonzini <pbonzini@...hat.com>,
        Xiao Guangrong <xiaoguangrong@...cent.com>,
        "lidongchen@...cent.com" <lidongchen@...cent.com>,
        "yongkaiwu@...cent.com" <yongkaiwu@...cent.com>
Subject: Re: [PATCH v2] mm: hwpoison: disable memory error handling on 1GB hugepage

On Tue, 11 Jun 2019 at 07:51, Naoya Horiguchi <n-horiguchi@...jp.nec.com> wrote:
>
> On Wed, May 29, 2019 at 04:31:01PM -0700, Mike Kravetz wrote:
> > 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@...cle.com> wrote:
> > >>
> > >> On 02/12/2018 06:48 PM, Michael Ellerman wrote:
> > >>> Andrew Morton <akpm@...ux-foundation.org> writes:
> > >>>
> > >>>> On Thu, 08 Feb 2018 12:30:45 +0000 Punit Agrawal <punit.agrawal@....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.
>
> Thanks for forwarding this to me, I'm feeling that memory error handling
> on 1GB hugepage is demanded as real use case.
>
> >
> > > 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.
>
> I agree that that's better.
>
> > 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.
>
> Yes, that's not practical, so we need at first establish the code base for
> 2GB hugetlb splitting and then extending it to 1GB next.

I'm working on this, thanks for the inputs.

Regards,
Wanpeng Li

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