From: David Hildenbrand <david@redhat.com>
To: Dan Williams <dan.j.williams@intel.com>,
Michal Hocko <mhocko@kernel.org>
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Subject: Re: [PATCH v1 04/10] vfio/type1: Prepare is_invalid_reserved_pfn() for PG_reserved changes
Date: Fri, 8 Nov 2019 11:21:40 +0100 [thread overview]
Message-ID: <931cb766-c3fb-8093-d8d0-144d328e69fc@redhat.com> (raw)
In-Reply-To: <a514e23c-616a-b093-ede5-b2c2c558a1a0@redhat.com>
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.
--
Thanks,
David / dhildenb
next prev parent reply other threads:[~2019-11-08 10:22 UTC|newest]
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 [this message]
2019-11-08 18:29 ` Dan Williams
2019-11-08 23:01 ` David Hildenbrand
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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