linux-kernel - Re: [PATCH v8 1/8] mm/memfd: Introduce userspace inaccessible memfd

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Date:   Fri, 23 Sep 2022 03:53:19 +0300
From:   "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     "Wang, Wei W" <wei.w.wang@...el.com>,
        Chao Peng <chao.p.peng@...ux.intel.com>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        "linux-api@...r.kernel.org" <linux-api@...r.kernel.org>,
        "linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
        "qemu-devel@...gnu.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@...nel.org" <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>,
        "Lutomirski, Andy" <luto@...nel.org>,
        "Nakajima, Jun" <jun.nakajima@...el.com>,
        "Hansen, Dave" <dave.hansen@...el.com>,
        "ak@...ux.intel.com" <ak@...ux.intel.com>,
        "david@...hat.com" <david@...hat.com>,
        "aarcange@...hat.com" <aarcange@...hat.com>,
        "ddutile@...hat.com" <ddutile@...hat.com>,
        "dhildenb@...hat.com" <dhildenb@...hat.com>,
        Quentin Perret <qperret@...gle.com>,
        Michael Roth <michael.roth@....com>,
        "Hocko, Michal" <mhocko@...e.com>,
        Muchun Song <songmuchun@...edance.com>
Subject: Re: [PATCH v8 1/8] mm/memfd: Introduce userspace inaccessible memfd

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