linuxppc-dev.lists.ozlabs.org archive mirror
 help / color / mirror / Atom feed
From: David Hildenbrand <david@redhat.com>
To: Sean Christopherson <sean.j.christopherson@intel.com>
Cc: linux-hyperv@vger.kernel.org, "Michal Hocko" <mhocko@suse.com>,
	"Radim Krčmář" <rkrcmar@redhat.com>,
	"KVM list" <kvm@vger.kernel.org>,
	"Pavel Tatashin" <pavel.tatashin@microsoft.com>,
	"KarimAllah Ahmed" <karahmed@amazon.de>,
	"Dave Hansen" <dave.hansen@linux.intel.com>,
	"Alexander Duyck" <alexander.duyck@gmail.com>,
	"Michal Hocko" <mhocko@kernel.org>,
	"Linux MM" <linux-mm@kvack.org>,
	"Paul Mackerras" <paulus@samba.org>,
	"H. Peter Anvin" <hpa@zytor.com>,
	"Wanpeng Li" <wanpengli@tencent.com>,
	"Alexander Duyck" <alexander.h.duyck@linux.intel.com>,
	"K. Y. Srinivasan" <kys@microsoft.com>,
	"Boris Ostrovsky" <boris.ostrovsky@oracle.com>,
	"Kees Cook" <keescook@chromium.org>,
	devel@driverdev.osuosl.org,
	"Stefano Stabellini" <sstabellini@kernel.org>,
	"Stephen Hemminger" <sthemmin@microsoft.com>,
	"Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com>,
	"Joerg Roedel" <joro@8bytes.org>, "X86 ML" <x86@kernel.org>,
	YueHaibing <yuehaibing@huawei.com>,
	"Matthew Wilcox (Oracle)" <willy@infradead.org>,
	"Mike Rapoport" <rppt@linux.ibm.com>,
	"Peter Zijlstra" <peterz@infradead.org>,
	"Ingo Molnar" <mingo@redhat.com>,
	"Vlastimil Babka" <vbabka@suse.cz>,
	"Anthony Yznaga" <anthony.yznaga@oracle.com>,
	"Oscar Salvador" <osalvador@suse.de>,
	"Isaac J. Manjarres" <isaacm@codeaurora.org>,
	"Matt Sickler" <Matt.Sickler@daktronics.com>,
	"Juergen Gross" <jgross@suse.com>,
	"Anshuman Khandual" <anshuman.khandual@arm.com>,
	"Haiyang Zhang" <haiyangz@microsoft.com>,
	"Sasha Levin" <sashal@kernel.org>,
	kvm-ppc@vger.kernel.org, "Qian Cai" <cai@lca.pw>,
	"Alex Williamson" <alex.williamson@redhat.com>,
	"Mike Rapoport" <rppt@linux.vnet.ibm.com>,
	"Borislav Petkov" <bp@alien8.de>,
	"Nicholas Piggin" <npiggin@gmail.com>,
	"Andy Lutomirski" <luto@kernel.org>,
	xen-devel <xen-devel@lists.xenproject.org>,
	"Dan Williams" <dan.j.williams@intel.com>,
	"Vitaly Kuznetsov" <vkuznets@redhat.com>,
	"Allison Randal" <allison@lohutok.net>,
	"Jim Mattson" <jmattson@google.com>,
	"Mel Gorman" <mgorman@techsingularity.net>,
	"Adam Borowski" <kilobyte@angband.pl>,
	"Cornelia Huck" <cohuck@redhat.com>,
	"Pavel Tatashin" <pasha.tatashin@soleen.com>,
	"Linux Kernel Mailing List" <linux-kernel@vger.kernel.org>,
	"Thomas Gleixner" <tglx@linutronix.de>,
	"Johannes Weiner" <hannes@cmpxchg.org>,
	"Paolo Bonzini" <pbonzini@redhat.com>,
	"Andrew Morton" <akpm@linux-foundation.org>,
	linuxppc-dev <linuxppc-dev@lists.ozlabs.org>
Subject: Re: [PATCH v1 03/10] KVM: Prepare kvm_is_reserved_pfn() for PG_reserved changes
Date: Tue, 5 Nov 2019 21:30:53 +0100	[thread overview]
Message-ID: <ed89cd61-7c45-8c9c-ffeb-f27b1872bd7a@redhat.com> (raw)
In-Reply-To: <20191105160000.GC8128@linux.intel.com>

>>> I think I know what's going wrong:
>>>
>>> Pages that are pinned via gfn_to_pfn() and friends take a references,
>>> however are often released via
>>> kvm_release_pfn_clean()/kvm_release_pfn_dirty()/kvm_release_page_clean()...
>>>
>>>
>>> E.g., in arch/x86/kvm/x86.c:reexecute_instruction()
>>>
>>> ...
>>> pfn = gfn_to_pfn(vcpu->kvm, gpa_to_gfn(gpa));
>>> ...
>>> kvm_release_pfn_clean(pfn);
>>>
>>>
>>>
>>> void kvm_release_pfn_clean(kvm_pfn_t pfn)
>>> {
>>> 	if (!is_error_noslot_pfn(pfn) && !kvm_is_reserved_pfn(pfn))
>>> 		put_page(pfn_to_page(pfn));
>>> }
>>>
>>> This function makes perfect sense as the counterpart for kvm_get_pfn():
>>>
>>> void kvm_get_pfn(kvm_pfn_t pfn)
>>> {
>>> 	if (!kvm_is_reserved_pfn(pfn))
>>> 		get_page(pfn_to_page(pfn));
>>> }
>>>
>>>
>>> As all ZONE_DEVICE pages are currently reserved, pages pinned via
>>> gfn_to_pfn() and friends will often not see a put_page() AFAIKS.
> 
> Assuming gup() takes a reference for ZONE_DEVICE pages, yes, this is a
> KVM bug.

Yes, it does take a reference AFAIKs. E.g.,

mm/gup.c:gup_pte_range():
...
		if (pte_devmap(pte)) {
			if (unlikely(flags & FOLL_LONGTERM))
				goto pte_unmap;

			pgmap = get_dev_pagemap(pte_pfn(pte), pgmap);
			if (unlikely(!pgmap)) {
				undo_dev_pagemap(nr, nr_start, pages);
				goto pte_unmap;
			}
		} else if (pte_special(pte))
			goto pte_unmap;

		VM_BUG_ON(!pfn_valid(pte_pfn(pte)));
		page = pte_page(pte);

		head = try_get_compound_head(page, 1);

try_get_compound_head() will increment the reference count.

> 
>>> Now, my patch does not change that, the result of
>>> kvm_is_reserved_pfn(pfn) will be unchanged. A proper fix for that would
>>> probably be
>>>
>>> a) To drop the reference to ZONE_DEVICE pages in gfn_to_pfn() and
>>> friends, after you successfully pinned the pages. (not sure if that's
>>> the right thing to do but you're the expert)
>>>
>>> b) To not use kvm_release_pfn_clean() and friends on pages that were
>>> definitely pinned.
> 
> This is already KVM's intent, i.e. the purpose of the PageReserved() check
> is simply to avoid putting a non-existent reference.  The problem is that
> KVM assumes pages with PG_reserved set are never pinned, which AFAICT was
> true when the code was first added.
> 
>> (talking to myself, sorry)
>>
>> Thinking again, dropping this patch from this series could effectively also
>> fix that issue. E.g., kvm_release_pfn_clean() and friends would always do a
>> put_page() if "pfn_valid() and !PageReserved()", so after patch 9 also on
>> ZONDE_DEVICE pages.
> 
> Yeah, this appears to be the correct fix.
> 
>> But it would have side effects that might not be desired. E.g.,:
>>
>> 1. kvm_pfn_to_page() would also return ZONE_DEVICE pages (might even be the
>> right thing to do).
> 
> This should be ok, at least on x86.  There are only three users of
> kvm_pfn_to_page().  Two of those are on allocations that are controlled by
> KVM and are guaranteed to be vanilla MAP_ANONYMOUS.  The third is on guest
> memory when running a nested guest, and in that case supporting ZONE_DEVICE
> memory is desirable, i.e. KVM should play nice with a guest that is backed
> by ZONE_DEVICE memory.
> 
>> 2. kvm_set_pfn_dirty() would also set ZONE_DEVICE pages dirty (might be
>> okay)
> 
> This is ok from a KVM perspective.

What about

void kvm_get_pfn(kvm_pfn_t pfn)
{
	if (!kvm_is_reserved_pfn(pfn))
		get_page(pfn_to_page(pfn));
}

Is a pure get_page() sufficient in case of ZONE_DEVICE?
(asking because of the live references obtained via 
get_dev_pagemap(pte_pfn(pte), pgmap) in mm/gup.c:gup_pte_range() 
somewhat confuse me :) )


> 
> The scarier code (for me) is transparent_hugepage_adjust() and
> kvm_mmu_zap_collapsible_spte(), as I don't at all understand the
> interaction between THP and _PAGE_DEVMAP.

The x86 KVM MMU code is one of the ugliest code I know (sorry, but it 
had to be said :/ ). Luckily, this should be independent of the 
PG_reserved thingy AFAIKs.

-- 

Thanks,

David / dhildenb


  reply	other threads:[~2019-11-05 21:40 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 [this message]
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
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

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=ed89cd61-7c45-8c9c-ffeb-f27b1872bd7a@redhat.com \
    --to=david@redhat.com \
    --cc=Matt.Sickler@daktronics.com \
    --cc=akpm@linux-foundation.org \
    --cc=alex.williamson@redhat.com \
    --cc=alexander.duyck@gmail.com \
    --cc=alexander.h.duyck@linux.intel.com \
    --cc=allison@lohutok.net \
    --cc=aneesh.kumar@linux.ibm.com \
    --cc=anshuman.khandual@arm.com \
    --cc=anthony.yznaga@oracle.com \
    --cc=boris.ostrovsky@oracle.com \
    --cc=bp@alien8.de \
    --cc=cai@lca.pw \
    --cc=cohuck@redhat.com \
    --cc=dan.j.williams@intel.com \
    --cc=dave.hansen@linux.intel.com \
    --cc=devel@driverdev.osuosl.org \
    --cc=haiyangz@microsoft.com \
    --cc=hannes@cmpxchg.org \
    --cc=hpa@zytor.com \
    --cc=isaacm@codeaurora.org \
    --cc=jgross@suse.com \
    --cc=jmattson@google.com \
    --cc=joro@8bytes.org \
    --cc=karahmed@amazon.de \
    --cc=keescook@chromium.org \
    --cc=kilobyte@angband.pl \
    --cc=kvm-ppc@vger.kernel.org \
    --cc=kvm@vger.kernel.org \
    --cc=kys@microsoft.com \
    --cc=linux-hyperv@vger.kernel.org \
    --cc=linux-kernel@vger.kernel.org \
    --cc=linux-mm@kvack.org \
    --cc=linuxppc-dev@lists.ozlabs.org \
    --cc=luto@kernel.org \
    --cc=mgorman@techsingularity.net \
    --cc=mhocko@kernel.org \
    --cc=mhocko@suse.com \
    --cc=mingo@redhat.com \
    --cc=npiggin@gmail.com \
    --cc=osalvador@suse.de \
    --cc=pasha.tatashin@soleen.com \
    --cc=paulus@samba.org \
    --cc=pavel.tatashin@microsoft.com \
    --cc=pbonzini@redhat.com \
    --cc=peterz@infradead.org \
    --cc=rkrcmar@redhat.com \
    --cc=rppt@linux.ibm.com \
    --cc=rppt@linux.vnet.ibm.com \
    --cc=sashal@kernel.org \
    --cc=sean.j.christopherson@intel.com \
    --cc=sstabellini@kernel.org \
    --cc=sthemmin@microsoft.com \
    --cc=tglx@linutronix.de \
    --cc=vbabka@suse.cz \
    --cc=vkuznets@redhat.com \
    --cc=wanpengli@tencent.com \
    --cc=willy@infradead.org \
    --cc=x86@kernel.org \
    --cc=xen-devel@lists.xenproject.org \
    --cc=yuehaibing@huawei.com \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).