linux-kernel - Re: [RFC, PATCHv1 00/28] 5-level paging

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Date:   Fri, 9 Dec 2016 08:40:11 -0800
From:   Andi Kleen <ak@...ux.intel.com>
To:     "Kirill A. Shutemov" <kirill@...temov.name>
Cc:     Ingo Molnar <mingo@...nel.org>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>, x86@...nel.org,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Arnd Bergmann <arnd@...db.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Andy Lutomirski <luto@...capital.net>,
        linux-arch@...r.kernel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC, PATCHv1 00/28] 5-level paging

> On other hand, large virtual address space would put more pressure on
> cache -- at least one more page table per process, if we make 56-bit VA
> default.

The top level page always has to be there unless you disable it at boot time
(unless you go for a scheme where some processes share top level pages, and
others do not, which would likely be very complicated)

But even with that it is more than one: A typical set up has at least two extra
4K pages overhead, one for the bottom and one for the top mappings. Could easily be
more.

-Andi