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Date:   Mon, 7 May 2018 05:23:46 -0700
From:   Matthew Wilcox <willy@infradead.org>
To:     "Kirill A. Shutemov" <kirill@shutemov.name>
Cc:     Andy Lutomirski <luto@amacapital.net>,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Michal Hocko <mhocko@kernel.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        X86 ML <x86@kernel.org>, Linux-MM <linux-mm@kvack.org>,
        LKML <linux-kernel@vger.kernel.org>
Subject: Re: Proof-of-concept: better(?) page-table manipulation API
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On Mon, May 07, 2018 at 02:31:25PM +0300, Kirill A. Shutemov wrote:
> > Also, what does lvl == 0 mean?  Is it the top or the bottom?  I think a
> > comment would be helpful.
> 
> It is bottom. But it should be up to architecture to decide.

That's not true because ...

> > > +static inline void ptp_walk(ptp_t *ptp, unsigned long addr)
> > > +{
> > > +       ptp->ptr = (unsigned long *)ptp_page_vaddr(ptp);
> > > +       ptp->ptr += __pt_index(addr, --ptp->lvl);
> > > +}
> > 
> > Can you add a comment that says what this function does?
> 
> Okay, I will.
> 
> > Why does it not change the level?
> 
> It does. --ptp->lvl.

... you've hardcoded that walking down decrements the level by 1.

I don't see that as a defect; it's just part of the API that needs
documenting.