linux-kernel - Re: [tip:x86/mm] x86/boot/compressed/64: Describe the logic behind the LA57 check

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Message-ID: <CA+55aFya-fZQVi45axVpv4PF_b8yhHfDxx3xCTX+gbN5Ec9VUQ@mail.gmail.com>
Date:   Mon, 12 Mar 2018 10:12:38 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Ingo Molnar <mingo@...nel.org>,
        "Kirill A. Shutemov" <kirill@...temov.name>,
        Peter Zijlstra <peterz@...radead.org>,
        Cyrill Gorcunov <gorcunov@...nvz.org>,
        Kees Cook <keescook@...omium.org>,
        Matthew Wilcox <willy@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Borislav Petkov <bp@...e.de>,
        Andy Shevchenko <andy.shevchenko@...il.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Peter Anvin <hpa@...or.com>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Jürgen Groß <jgross@...e.com>,
        linux-tip-commits@...r.kernel.org,
        Dave Hansen <dave.hansen@...el.com>
Subject: Re: [tip:x86/mm] x86/boot/compressed/64: Describe the logic behind
 the LA57 check

On Mon, Mar 12, 2018 at 10:06 AM, Andy Lutomirski <luto@...nel.org> wrote:
>
> I'd be surprised if there's a noticeable performance hit on anything
> except the micro-est of benchmarks.  We're talking one extra
> intermediate paging structure cache entry in use, maybe a few data
> cache lines, and (wild guess) 0 extra cycles on a TLB miss in the
> normal case.  This is because the walks are almost never going to
> start at the root.

Probably. But VM people may disagree if they already have high TLB miss costs.

> The real hit will be the extra page table for every task.

.. and it's unclear how noticeable that might be. It's not like it's
per-thread, only per process, and very few people have so many
processes that a page per process matters.

But regardless, I think we're better off with a "wait and see" approach.

IOW, try to use 5-level whenever possible for now, and _if_ somebody
actually can show that 4-level page tables perform better or have some
other advantage, we can then try to be clever later when it's all
tested and it's just an optimization, not a "that code won't even run
normally and gets basically zero coverage".

                Linus