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Date:   Fri, 23 Sep 2022 03:53:19 +0300
From:   "Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>
To:     Sean Christopherson <seanjc@google.com>
Cc:     "Wang, Wei W" <wei.w.wang@intel.com>,
        Chao Peng <chao.p.peng@linux.intel.com>,
        "kvm@vger.kernel.org" <kvm@vger.kernel.org>,
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        Jonathan Corbet <corbet@lwn.net>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        Wanpeng Li <wanpengli@tencent.com>,
        Jim Mattson <jmattson@google.com>,
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        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
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        "H . Peter Anvin" <hpa@zytor.com>, Hugh Dickins <hughd@google.com>,
        Jeff Layton <jlayton@kernel.org>,
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        Shuah Khan <shuah@kernel.org>, Mike Rapoport <rppt@kernel.org>,
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        "Maciej S . Szmigiero" <mail@maciej.szmigiero.name>,
        Vlastimil Babka <vbabka@suse.cz>,
        Vishal Annapurve <vannapurve@google.com>,
        Yu Zhang <yu.c.zhang@linux.intel.com>,
        "Lutomirski, Andy" <luto@kernel.org>,
        "Nakajima, Jun" <jun.nakajima@intel.com>,
        "Hansen, Dave" <dave.hansen@intel.com>,
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        Muchun Song <songmuchun@bytedance.com>
Subject: Re: [PATCH v8 1/8] mm/memfd: Introduce userspace inaccessible memfd
Message-ID: <20220923005319.wkzpl36uailh4zbw@box.shutemov.name>
References: <20220915142913.2213336-1-chao.p.peng@linux.intel.com>
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On Thu, Sep 22, 2022 at 07:49:18PM +0000, Sean Christopherson wrote:
> On Thu, Sep 22, 2022, Wang, Wei W wrote:
> > On Thursday, September 15, 2022 10:29 PM, Chao Peng wrote:
> > > +int inaccessible_get_pfn(struct file *file, pgoff_t offset, pfn_t *pfn,
> > > +			 int *order)
> > 
> > Better to remove "order" from this interface?
> 
> Hard 'no'.
> 
> > Some callers only need to get pfn, and no need to bother with
> > defining and inputting something unused. For callers who need the "order",
> > can easily get it via thp_order(pfn_to_page(pfn)) on their own.
> 
> That requires (a) assuming the pfn is backed by struct page, and (b) assuming the
> struct page is a transparent huge page.  That might be true for the current
> implementation, but it most certainly will not always be true.
> 
> KVM originally did things like this, where there was dedicated code for THP vs.
> HugeTLB, and it was a mess.  The goal here is very much to avoid repeating those
> mistakes.  Have the backing store _tell_ KVM how big the mapping is, don't force
> KVM to rediscover the info on its own.

I guess we can allow order pointer to be NULL to cover caller that don't
need to know the order. Is it useful?

-- 
  Kiryl Shutsemau / Kirill A. Shutemov