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

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Message-ID: <CA+EHjTy+2JG_=Xw5ojwZAW6+3Nr4-4a8YCBxWN5PmAhh7jjT+A@mail.gmail.com>
Date:   Fri, 23 Sep 2022 16:20:21 +0100
From:   Fuad Tabba <tabba@...gle.com>
To:     "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>
Cc:     Sean Christopherson <seanjc@...gle.com>,
        "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>,
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        Paolo Bonzini <pbonzini@...hat.com>,
        Jonathan Corbet <corbet@....net>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
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        Muchun Song <songmuchun@...edance.com>
Subject: Re: [PATCH v8 1/8] mm/memfd: Introduce userspace inaccessible memfd

Hi,

On Fri, Sep 23, 2022 at 1:53 AM Kirill A . Shutemov
<kirill.shutemov@...ux.intel.com> wrote:
>
> 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?

I think that would be useful. In pKVM we don't need to know the order,
and I had to use a dummy variable when porting V7.

Cheers,
/fuad


> --
>   Kiryl Shutsemau / Kirill A. Shutemov