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From: David Hildenbrand <david@redhat.com>
To: Dan Williams <dan.j.williams@intel.com>
Cc: linux-hyperv@vger.kernel.org, "Michal Hocko" <mhocko@suse.com>,
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Subject: Re: [PATCH v1 04/10] vfio/type1: Prepare is_invalid_reserved_pfn() for PG_reserved changes
Date: Sat, 9 Nov 2019 00:01:40 +0100
Message-ID: <5413f9c3-dc6a-d8ba-3777-41e99714af3d@redhat.com> (raw)
In-Reply-To: <CAPcyv4ivq=EPUePXiX2ErcVyF7+dV9Yv215Oue7X_Y2X_Jfw8Q@mail.gmail.com>

On 08.11.19 19:29, Dan Williams wrote:
> On Fri, Nov 8, 2019 at 2:22 AM David Hildenbrand <david@redhat.com> wrote:
>>
>> On 08.11.19 08:14, David Hildenbrand wrote:
>>> On 08.11.19 06:09, Dan Williams wrote:
>>>> On Thu, Nov 7, 2019 at 2:07 PM David Hildenbrand <david@redhat.com> wrote:
>>>>>
>>>>> On 07.11.19 19:22, David Hildenbrand wrote:
>>>>>>
>>>>>>
>>>>>>> Am 07.11.2019 um 16:40 schrieb Dan Williams <dan.j.williams@intel.com>:
>>>>>>>
>>>>>>> On Thu, Oct 24, 2019 at 5:12 AM David Hildenbrand <david@redhat.com> wrote:
>>>>>>>>
>>>>>>>> Right now, ZONE_DEVICE memory is always set PG_reserved. We want to
>>>>>>>> change that.
>>>>>>>>
>>>>>>>> KVM has this weird use case that you can map anything from /dev/mem
>>>>>>>> into the guest. pfn_valid() is not a reliable check whether the memmap
>>>>>>>> was initialized and can be touched. pfn_to_online_page() makes sure
>>>>>>>> that we have an initialized memmap (and don't have ZONE_DEVICE memory).
>>>>>>>>
>>>>>>>> Rewrite is_invalid_reserved_pfn() similar to kvm_is_reserved_pfn() to make
>>>>>>>> sure the function produces the same result once we stop setting ZONE_DEVICE
>>>>>>>> pages PG_reserved.
>>>>>>>>
>>>>>>>> Cc: Alex Williamson <alex.williamson@redhat.com>
>>>>>>>> Cc: Cornelia Huck <cohuck@redhat.com>
>>>>>>>> Signed-off-by: David Hildenbrand <david@redhat.com>
>>>>>>>> ---
>>>>>>>> drivers/vfio/vfio_iommu_type1.c | 10 ++++++++--
>>>>>>>> 1 file changed, 8 insertions(+), 2 deletions(-)
>>>>>>>>
>>>>>>>> diff --git a/drivers/vfio/vfio_iommu_type1.c b/drivers/vfio/vfio_iommu_type1.c
>>>>>>>> index 2ada8e6cdb88..f8ce8c408ba8 100644
>>>>>>>> --- a/drivers/vfio/vfio_iommu_type1.c
>>>>>>>> +++ b/drivers/vfio/vfio_iommu_type1.c
>>>>>>>> @@ -299,9 +299,15 @@ static int vfio_lock_acct(struct vfio_dma *dma, long npage, bool async)
>>>>>>>>      */
>>>>>>>> static bool is_invalid_reserved_pfn(unsigned long pfn)
>>>>>>>> {
>>>>>>>> -       if (pfn_valid(pfn))
>>>>>>>> -               return PageReserved(pfn_to_page(pfn));
>>>>>>>> +       struct page *page = pfn_to_online_page(pfn);
>>>>>>>
>>>>>>> Ugh, I just realized this is not a safe conversion until
>>>>>>> pfn_to_online_page() is moved over to subsection granularity. As it
>>>>>>> stands it will return true for any ZONE_DEVICE pages that share a
>>>>>>> section with boot memory.
>>>>>>
>>>>>> That should not happen right now and I commented back when you introduced subsection support that I don’t want to have ZONE_DEVICE mixed with online pages in a section. Having memory block devices that partially span ZONE_DEVICE would be ... really weird. With something like pfn_active() - as discussed - we could at least make this check work - but I am not sure if we really want to go down that path. In the worst case, some MB of RAM are lost ... I guess this needs more thought.
>>>>>>
>>>>>
>>>>> I just realized the "boot memory" part. Is that a real thing? IOW, can
>>>>> we have ZONE_DEVICE falling into a memory block (with holes)? I somewhat
>>>>> have doubts that this would work ...
>>>>
>>>> One of the real world failure cases that started the subsection effect
>>>> is that Persistent Memory collides with System RAM on a 64MB boundary
>>>> on shipping platforms. System RAM ends on a 64MB boundary and due to a
>>>> lack of memory controller resources PMEM is mapped contiguously at the
>>>> end of that boundary. Some more details in the subsection cover letter
>>>> / changelogs [1] [2]. It's not sufficient to just lose some memory,
>>>> that's the broken implementation that lead to the subsection work
>>>> because the lost memory may change from one boot to the next and
>>>> software can't reliably inject a padding that conforms to the x86
>>>> 128MB section constraint.
>>>
>>> Thanks, I thought it was mostly for weird alignment where other parts of
>>> the section are basically "holes" and not memory.
>>>
>>> Yes, it is a real bug that ZONE_DEVICE pages fall into sections that are
>>> marked SECTION_IS_ONLINE.
>>>
>>>>
>>>> Suffice to say I think we need your pfn_active() to get subsection
>>>> granularity pfn_to_online_page() before PageReserved() can be removed.
>>>
>>> I agree that we have to fix this. I don't like ZONE_DEVICE pages falling
>>> into memory device blocks (e.g., cannot get offlined), but I guess that
>>> train is gone :) As long as it's not for memory hotplug, I can most
>>> probably live with this.
>>>
>>> Also, I'd like to get Michals opinion on this and the pfn_active()
>>> approach, but I can understand he's busy.
>>>
>>> This patch set can wait, I won't be working next week besides
>>> reading/writing mails either way.
>>>
>>> Is anybody looking into the pfn_active() thingy?
>>>
>>
>> I wonder if we should do something like this right now to fix this
>> (exclude the false positive ZONE_DEVICE pages we could have within an
>> online section, which was not possible before subsection hotplug):
>>
>> diff --git a/include/linux/memory_hotplug.h b/include/linux/memory_hotplug.h
>> index 384ffb3d69ab..490a9e9358b3 100644
>> --- a/include/linux/memory_hotplug.h
>> +++ b/include/linux/memory_hotplug.h
>> @@ -30,6 +30,8 @@ struct vmem_altmap;
>>           if (___nr < NR_MEM_SECTIONS && online_section_nr(___nr) && \
>>               pfn_valid_within(___pfn))                              \
>>                   ___page = pfn_to_page(___pfn);                     \
>> +       if (unlikely(___page && is_zone_device_page(___page)))     \
>> +               ___page = NULL;                                    \
>>           ___page;                                                   \
>>    })
>>
>>
>> Yeah, it's another is_zone_device_page(), but it should not be racy
>> here, as we want to exclude, not include ZONE_DEVICE.
>>
>> I don't have time to look into this right now, unfortunately.
> 
> I don't want to band-aid without an actual bug report. I'll take a
> look at a subsection-map for the online state.
> 

Fair enough, but at least in what I proposed for pfn_active(), this 
check would exist in pfn_to_online_page() in a similar way - and it is 
certainly easier to backport. But yeah, triggering this might not be easy.

-- 

Thanks,

David / dhildenb


  reply index

Thread overview: 43+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-10-24 12:09 [PATCH v1 00/10] mm: Don't mark hotplugged pages PG_reserved (including ZONE_DEVICE) David Hildenbrand
2019-10-24 12:09 ` [PATCH v1 01/10] mm/memory_hotplug: Don't allow to online/offline memory blocks with holes David Hildenbrand
2019-11-05  1:30   ` Dan Williams
2019-11-05  9:31     ` David Hildenbrand
2019-10-24 12:09 ` [PATCH v1 02/10] KVM: x86/mmu: Prepare kvm_is_mmio_pfn() for PG_reserved changes David Hildenbrand
2019-11-05  1:37   ` Dan Williams
2019-11-05 11:09     ` David Hildenbrand
2019-10-24 12:09 ` [PATCH v1 03/10] KVM: Prepare kvm_is_reserved_pfn() " David Hildenbrand
2019-11-05  4:38   ` Dan Williams
2019-11-05  9:17     ` David Hildenbrand
2019-11-05  9:49       ` David Hildenbrand
2019-11-05 10:02         ` David Hildenbrand
2019-11-05 16:00           ` Sean Christopherson
2019-11-05 20:30             ` David Hildenbrand
2019-11-05 22:22               ` Sean Christopherson
2019-11-05 23:02               ` Dan Williams
2019-11-05 23:13                 ` Sean Christopherson
2019-11-05 23:30                   ` Dan Williams
2019-11-05 23:42                     ` Sean Christopherson
2019-11-05 23:43                     ` Dan Williams
2019-11-06  0:03                       ` Sean Christopherson
2019-11-06  0:08                         ` Dan Williams
2019-11-06  6:56                           ` David Hildenbrand
2019-11-06 16:09                             ` Sean Christopherson
2019-10-24 12:09 ` [PATCH v1 04/10] vfio/type1: Prepare is_invalid_reserved_pfn() " David Hildenbrand
2019-11-07 15:40   ` Dan Williams
2019-11-07 18:22     ` David Hildenbrand
2019-11-07 22:07       ` David Hildenbrand
2019-11-08  5:09         ` Dan Williams
2019-11-08  7:14           ` David Hildenbrand
2019-11-08 10:21             ` David Hildenbrand
2019-11-08 18:29               ` Dan Williams
2019-11-08 23:01                 ` David Hildenbrand [this message]
2019-10-24 12:09 ` [PATCH v1 05/10] powerpc/book3s: Prepare kvmppc_book3s_instantiate_page() " David Hildenbrand
2019-10-24 12:09 ` [PATCH v1 06/10] powerpc/64s: Prepare hash_page_do_lazy_icache() " David Hildenbrand
2019-10-24 12:09 ` [PATCH v1 07/10] powerpc/mm: Prepare maybe_pte_to_page() " David Hildenbrand
2019-10-24 12:09 ` [PATCH v1 08/10] x86/mm: Prepare __ioremap_check_ram() " David Hildenbrand
2019-10-24 12:09 ` [PATCH v1 09/10] mm/memory_hotplug: Don't mark pages PG_reserved when initializing the memmap David Hildenbrand
2019-11-04 22:44   ` Boris Ostrovsky
2019-11-05 10:18     ` David Hildenbrand
2019-11-05 16:06       ` Boris Ostrovsky
2019-10-24 12:09 ` [PATCH v1 10/10] mm/usercopy.c: Update comment in check_page_span() regarding ZONE_DEVICE David Hildenbrand
2019-11-01 19:24 ` [PATCH v1 00/10] mm: Don't mark hotplugged pages PG_reserved (including ZONE_DEVICE) David Hildenbrand

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