linux-kernel - Re: [patch V163 27/51] x86/mm/pti: Populate user PGD

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Date:   Mon, 18 Dec 2017 12:45:13 -0800
From:   Dave Hansen <dave.hansen@...el.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>, x86@...nel.org,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andy Lutomirsky <luto@...nel.org>,
        Borislav Petkov <bpetkov@...e.de>,
        Greg KH <gregkh@...uxfoundation.org>, keescook@...gle.com,
        hughd@...gle.com, Brian Gerst <brgerst@...il.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Denys Vlasenko <dvlasenk@...hat.com>,
        Rik van Riel <riel@...hat.com>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Juergen Gross <jgross@...e.com>,
        David Laight <David.Laight@...lab.com>,
        Eduardo Valentin <eduval@...zon.com>, aliguori@...zon.com,
        Will Deacon <will.deacon@....com>, daniel.gruss@...k.tugraz.at,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Ingo Molnar <mingo@...nel.org>, Borislav Petkov <bp@...e.de>,
        "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [patch V163 27/51] x86/mm/pti: Populate user PGD

On 12/18/2017 12:41 PM, Peter Zijlstra wrote:
>> I also don't think the user_shared area of the fixmap can get *that*
>> big.  Does anybody know offhand what the theoretical limits are there?
> Problem there is the nr_cpus term I think, we currently have up to 8k
> CPUs, but I can see that getting bigger in the future.

It only matters if we go over 512GB, though.  Is the per-cpu part of the
fixmap ever more than 512GB/8k=64MB?

In any case, it's not hard to make a little loop.  I think the common
case, at least is that we only do 4 PGDs.