linux-kernel - Re: [PATCH v7 07/14] KVM: Use gfn instead of hva for mmu_notifier

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Date:   Tue, 19 Jul 2022 22:02:48 +0800
From:   Chao Peng <chao.p.peng@...ux.intel.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     "Gupta, Pankaj" <pankaj.gupta@....com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        linux-fsdevel@...r.kernel.org, linux-api@...r.kernel.org,
        linux-doc@...r.kernel.org, qemu-devel@...gnu.org,
        linux-kselftest@...r.kernel.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        Jonathan Corbet <corbet@....net>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        x86@...nel.org, "H . Peter Anvin" <hpa@...or.com>,
        Hugh Dickins <hughd@...gle.com>,
        Jeff Layton <jlayton@...nel.org>,
        "J . Bruce Fields" <bfields@...ldses.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Shuah Khan <shuah@...nel.org>, Mike Rapoport <rppt@...nel.org>,
        Steven Price <steven.price@....com>,
        "Maciej S . Szmigiero" <mail@...iej.szmigiero.name>,
        Vlastimil Babka <vbabka@...e.cz>,
        Vishal Annapurve <vannapurve@...gle.com>,
        Yu Zhang <yu.c.zhang@...ux.intel.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        luto@...nel.org, jun.nakajima@...el.com, dave.hansen@...el.com,
        ak@...ux.intel.com, david@...hat.com, aarcange@...hat.com,
        ddutile@...hat.com, dhildenb@...hat.com,
        Quentin Perret <qperret@...gle.com>,
        Michael Roth <michael.roth@....com>, mhocko@...e.com,
        Muchun Song <songmuchun@...edance.com>
Subject: Re: [PATCH v7 07/14] KVM: Use gfn instead of hva for
 mmu_notifier_retry

On Mon, Jul 18, 2022 at 03:26:34PM +0000, Sean Christopherson wrote:
> On Mon, Jul 18, 2022, Chao Peng wrote:
> > On Fri, Jul 15, 2022 at 01:36:15PM +0200, Gupta, Pankaj wrote:
> > > > Currently in mmu_notifier validate path, hva range is recorded and then
> > > > checked in the mmu_notifier_retry_hva() from page fault path. However
> > > > for the to be introduced private memory, a page fault may not have a hva
> > > 
> > > As this patch appeared in v7, just wondering did you see an actual bug
> > > because of it? And not having corresponding 'hva' occurs only with private
> > > memory because its not mapped to host userspace?
> > 
> > The addressed problem is not new in this version, previous versions I
> > also had code to handle it (just in different way). But the problem is:
> > mmu_notifier/memfile_notifier may be in the progress of invalidating a
> > pfn that obtained earlier in the page fault handler, when happens, we
> > should retry the fault. In v6 I used global mmu_notifier_retry() for
> > memfile_notifier but that can block unrelated mmu_notifer invalidation
> > which has hva range specified.
> > 
> > Sean gave a comment at https://lkml.org/lkml/2022/6/17/1001 to separate
> > memfile_notifier from mmu_notifier but during the implementation I
> > realized we actually can reuse the same code for shared and private
> > memory if both using gpa range and that can simplify the code handling
> > in kvm_zap_gfn_range and some other code (e.g. we don't need two
> > versions for memfile_notifier/mmu_notifier).
> 
> This should work, though I'm undecided as to whether or not it's a good idea.  KVM
> allows aliasing multiple gfns to a single hva, and so using the gfn could result
> in a much larger range being rejected given the simplistic algorithm for handling
> multiple ranges in kvm_inc_notifier_count().  But I assume such aliasing is uncommon,
> so I'm not sure it's worth optimizing for.

That can be a real problem for current v7 code, __kvm_handle_hva_range()
loops all possible gfn_range for a given hva_range but the
on_lock/on_unlock is invoked only once, this should work for hva_range,
but not gfn_range since we can have multiple of them.

> 
> > Adding gpa range for private memory invalidation also relieves the
> > above blocking issue between private memory page fault and mmu_notifier.