linux-kernel - Re: Question on the five-level page table support patches

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Message-ID: <fdc80e3c-6909-cf39-fe0b-6f1c012571e4@physik.fu-berlin.de>
Date:   Mon, 24 Apr 2017 22:37:40 +0200
From:   John Paul Adrian Glaubitz <glaubitz@...sik.fu-berlin.de>
To:     "Kirill A. Shutemov" <kirill@...temov.name>
Cc:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        linux-kernel@...r.kernel.org, Andi Kleen <ak@...ux.intel.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Andy Lutomirski <luto@...capital.net>,
        Michal Hocko <mhocko@...e.com>, linux-arch@...r.kernel.org,
        linux-mm@...ck.org
Subject: Re: Question on the five-level page table support patches

On 04/24/2017 06:19 PM, Kirill A. Shutemov wrote:
> In proposed implementation, we also use hint address, but in different
> way: by default, if hint address is NULL, kernel would not create mappings
> above 47-bits, preserving compatibility.

Ooooh, that would solve a lot of problems actually if it were to be available
on all architectures. On SPARC, the situation is really annoying and I have
been discussing a solution with the Qt developers and they suggested a
similar approach, just one that would also apply to brk() [1].

> If an application wants to have access to larger address space, it has to
> specify hint addess above 47-bits.
> 
> See details here:
> 
> http://lkml.kernel.org/r/20170420162147.86517-10-kirill.shutemov@linux.intel.com

Thanks. I'll have a read. Although from your message I'm reading out that
this particular proposal got rejected.

Would be really nice to able to have a canonical solution for this issue,
it's been biting us on SPARC for quite a while now due to the fact that
virtual address space has been 52 bits on SPARC for a while now.

Adrian

> [1] https://bugreports.qt.io/browse/QTBUG-56264

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