From: "Kirill A. Shutemov" <kirill@shutemov.name>
To: Andrea Arcangeli <aarcange@redhat.com>
Cc: Andrew Morton <akpm@linux-foundation.org>,
linux-kernel@vger.kernel.org, linux-mm@kvack.org,
qemu-devel@nongnu.org,
"Dr. David Alan Gilbert" <dgilbert@redhat.com>,
"Li, Liang Z" <liang.z.li@intel.com>,
Amit Shah <amit.shah@redhat.com>,
Paolo Bonzini <pbonzini@redhat.com>
Subject: Re: [PATCH 1/1] mm: thp: kvm: fix memory corruption in KVM with THP enabled
Date: Wed, 27 Apr 2016 18:18:34 +0300 [thread overview]
Message-ID: <20160427151834.GC22035@node.shutemov.name> (raw)
In-Reply-To: <20160427145957.GA9217@redhat.com>
On Wed, Apr 27, 2016 at 04:59:57PM +0200, Andrea Arcangeli wrote:
> On Wed, Apr 27, 2016 at 04:50:30PM +0300, Kirill A. Shutemov wrote:
> > I know nothing about kvm. How do you protect against pmd splitting between
> > get_user_pages() and the check?
>
> get_user_pages_fast() runs fully lockless and unpins the page right
> away (we need a get_user_pages_fast without the FOLL_GET in fact to
> avoid a totally useless atomic_inc/dec!).
>
> Then we take a lock that is also taken by
> mmu_notifier_invalidate_range_start. This way __split_huge_pmd will
> block in mmu_notifier_invalidate_range_start if it tries to run again
> (every other mmu notifier like mmu_notifier_invalidate_page will also
> block).
>
> Then after we serialized against __split_huge_pmd through the MMU
> notifier KVM internal locking, we are able to tell if any mmu_notifier
> invalidate happened in the region just before get_user_pages_fast()
> was invoked, until we call PageCompoundTransMap and we actually map
> the shadow pagetable into the compound page with hugepage
> granularity (to allow real 2MB TLBs if guest also uses trans_huge_pmd
> in the guest pagetables).
>
> After the shadow pagetable is mapped, we drop the internal MMU
> notifier lock and __split_huge_pmd mmu_notifier_invalidate_range_start
> can continue and drop the shadow pagetable that we just mapped in the
> above paragraph just before dropping the mmu notifier internal lock.
>
> To be able to tell if any invalidate happened while
> get_user_pages_fast was running and until we grab the lock again and
> we start mapping the shadow pagtable we use:
>
> mmu_seq = vcpu->kvm->mmu_notifier_seq;
> smp_rmb();
>
> if (try_async_pf(vcpu, prefault, gfn, v, &pfn, write, &map_writable))
> ^^^^^^^^^^^^ this is get_user_pages and does put_page on the page
> and just returns the &pfn
> this is why we need a get_user_pages_fast that won't
> attempt to touch the page->_count at all! we can avoid
> 2 atomic ops for each secondary MMU fault that way
> return 0;
>
> spin_lock(&vcpu->kvm->mmu_lock);
> if (mmu_notifier_retry(vcpu->kvm, mmu_seq))
> goto out_unlock;
> ... here we check PageTransCompoundMap(pfn_to_page(pfn)) and
> map a 4k or 2MB shadow pagetable on "pfn" ...
>
>
> Note mmu_notifier_retry does the other side of the smp_rmb():
>
> smp_rmb();
> if (kvm->mmu_notifier_seq != mmu_seq)
> return 1;
> return 0;
Okay, I see.
But do we really want to make PageTransCompoundMap() visiable beyond KVM
code? It looks like too KVM-specific.
--
Kirill A. Shutemov
next prev parent reply other threads:[~2016-04-27 15:18 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-04-27 12:04 [PATCH 1/1] mm: thp: kvm: fix memory corruption in KVM with THP enabled Andrea Arcangeli
2016-04-27 13:50 ` Kirill A. Shutemov
2016-04-27 14:59 ` Andrea Arcangeli
2016-04-27 15:18 ` Kirill A. Shutemov [this message]
2016-04-27 15:57 ` Andrea Arcangeli
2016-04-27 16:03 ` Andrea Arcangeli
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