linux-kernel - Re: [PATCH v6 6/8] KVM: Handle page fault for private memory

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Message-ID: <CAGtprH9knCr++C7jgXYCi1zfYcreip1uun-d+eucjEQy9xymNg@mail.gmail.com>
Date:   Wed, 20 Jul 2022 16:08:10 -0700
From:   Vishal Annapurve <vannapurve@...gle.com>
To:     Xiaoyao Li <xiaoyao.li@...el.com>
Cc:     Sean Christopherson <seanjc@...gle.com>,
        Michael Roth <michael.roth@....com>,
        Chao Peng <chao.p.peng@...ux.intel.com>,
        "Nikunj A. Dadhania" <nikunj@....com>,
        kvm list <kvm@...r.kernel.org>,
        LKML <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,
        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 <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>,
        Mike Rapoport <rppt@...nel.org>,
        Steven Price <steven.price@....com>,
        "Maciej S . Szmigiero" <mail@...iej.szmigiero.name>,
        Vlastimil Babka <vbabka@...e.cz>,
        Yu Zhang <yu.c.zhang@...ux.intel.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        Jun Nakajima <jun.nakajima@...el.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Andi Kleen <ak@...ux.intel.com>,
        David Hildenbrand <david@...hat.com>, aarcange@...hat.com,
        ddutile@...hat.com, dhildenb@...hat.com,
        Quentin Perret <qperret@...gle.com>, mhocko@...e.com
Subject: Re: [PATCH v6 6/8] KVM: Handle page fault for private memory

> > Hmm, so a new slot->arch.page_attr array shouldn't be necessary, KVM can instead
> > update slot->arch.lpage_info on shared<->private conversions.  Detecting whether
> > a given range is partially mapped could get nasty if KVM defers tracking to the
> > backing store, but if KVM itself does the tracking as was previously suggested[*],
> > then updating lpage_info should be relatively straightfoward, e.g. use
> > xa_for_each_range() to see if a given 2mb/1gb range is completely covered (fully
> > shared) or not covered at all (fully private).
> >
> > [*] https://lore.kernel.org/all/YofeZps9YXgtP3f1@google.com
>
> Yes, slot->arch.page_attr was introduced to help identify whether a page
> is completely shared/private at given level. It seems XARRAY can serve
> the same purpose, though I know nothing about it. Looking forward to
> seeing the patch of using XARRAY.
>
> yes, update slot->arch.lpage_info is good to utilize the existing logic
> and Isaku has applied it to slot->arch.lpage_info for 2MB support patches.

Chao, are you planning to implement these changes to ensure proper
handling of hugepages partially mapped as private/shared in subsequent
versions of this series?
Or is this something left to be handled by the architecture specific code?

Regards,
Vishal