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<931cb766-c3fb-8093-d8d0-144d328e69fc@redhat.com> In-Reply-To: <931cb766-c3fb-8093-d8d0-144d328e69fc@redhat.com> From: Dan Williams Date: Fri, 8 Nov 2019 10:29:06 -0800 Message-ID: Subject: Re: [PATCH v1 04/10] vfio/type1: Prepare is_invalid_reserved_pfn() for PG_reserved changes To: David Hildenbrand Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable X-Mailman-Approved-At: Sat, 09 Nov 2019 12:19:43 +1100 X-BeenThere: linuxppc-dev@lists.ozlabs.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: linux-hyperv@vger.kernel.org, Michal Hocko , =?UTF-8?B?UmFkaW0gS3LEjW3DocWZ?= , KVM list , Pavel Tatashin , KarimAllah Ahmed , Dave Hansen , Alexander Duyck , Michal Hocko , Linux MM , Paul Mackerras , "H. Peter Anvin" , Wanpeng Li , Alexander Duyck , "K. Y. Srinivasan" , Thomas Gleixner , Kees Cook , devel@driverdev.osuosl.org, Stefano Stabellini , Stephen Hemminger , "Aneesh Kumar K.V" , Joerg Roedel , X86 ML , YueHaibing , "Matthew Wilcox \(Oracle\)" , Mike Rapoport , Peter Zijlstra , Ingo Molnar , Vlastimil Babka , Anthony Yznaga , Oscar Salvador , "Isaac J. Manjarres" , Matt Sickler , Juergen Gross , Anshuman Khandual , Haiyang Zhang , Sasha Levin , kvm-ppc@vger.kernel.org, Qian Cai , Alex Williamson , Mike Rapoport , Borislav Petkov , Nicholas Piggin , Andy Lutomirski , xen-devel , Boris Ostrovsky , Vitaly Kuznetsov , Allison Randal , Jim Mattson , Mel Gorman , Cornelia Huck , Pavel Tatashin , Linux Kernel Mailing List , Sean Christopherson , Johannes Weiner , Paolo Bonzini , Andrew Morton , linuxppc-dev Errors-To: linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@lists.ozlabs.org Sender: "Linuxppc-dev" On Fri, Nov 8, 2019 at 2:22 AM David Hildenbrand 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 wr= ote: > >>> > >>> On 07.11.19 19:22, David Hildenbrand wrote: > >>>> > >>>> > >>>>> Am 07.11.2019 um 16:40 schrieb Dan Williams : > >>>>> > >>>>> =EF=BB=BFOn Thu, Oct 24, 2019 at 5:12 AM David Hildenbrand wrote: > >>>>>> > >>>>>> Right now, ZONE_DEVICE memory is always set PG_reserved. We want t= o > >>>>>> change that. > >>>>>> > >>>>>> KVM has this weird use case that you can map anything from /dev/me= m > >>>>>> into the guest. pfn_valid() is not a reliable check whether the me= mmap > >>>>>> was initialized and can be touched. pfn_to_online_page() makes sur= e > >>>>>> that we have an initialized memmap (and don't have ZONE_DEVICE mem= ory). > >>>>>> > >>>>>> Rewrite is_invalid_reserved_pfn() similar to kvm_is_reserved_pfn()= to make > >>>>>> sure the function produces the same result once we stop setting ZO= NE_DEVICE > >>>>>> pages PG_reserved. > >>>>>> > >>>>>> Cc: Alex Williamson > >>>>>> Cc: Cornelia Huck > >>>>>> Signed-off-by: David Hildenbrand > >>>>>> --- > >>>>>> 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_i= ommu_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 *dm= a, 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 =3D 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 intro= duced subsection support that I don=E2=80=99t want to have ZONE_DEVICE mixe= d with online pages in a section. Having memory block devices that partiall= y span ZONE_DEVICE would be ... really weird. With something like pfn_activ= e() - 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, ca= n > >>> we have ZONE_DEVICE falling into a memory block (with holes)? I somew= hat > >>> 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 o= f > > the section are basically "holes" and not memory. > > > > Yes, it is a real bug that ZONE_DEVICE pages fall into sections that ar= e > > 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 fallin= g > > 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_hotplu= g.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 =3D pfn_to_page(___pfn); \ > + if (unlikely(___page && is_zone_device_page(___page))) \ > + ___page =3D 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.