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From:   Fuad Tabba <tabba@google.com>
Date:   Fri, 23 Sep 2022 16:20:21 +0100
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Subject: Re: [PATCH v8 1/8] mm/memfd: Introduce userspace inaccessible memfd
To:     "Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>
Cc:     Sean Christopherson <seanjc@google.com>,
        "Wang, Wei W" <wei.w.wang@intel.com>,
        Chao Peng <chao.p.peng@linux.intel.com>,
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Hi,

On Fri, Sep 23, 2022 at 1:53 AM Kirill A . Shutemov
<kirill.shutemov@linux.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