linux-kernel - Re: [PATCH RFC v3 1/2] mm: Add personality flag to limit address to 47 bits

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Message-ID: <ZuKIMz7U8rDrq8jA@ghost>
Date: Wed, 11 Sep 2024 23:20:35 -0700
From: Charlie Jenkins <charlie@...osinc.com>
To: Michael Ellerman <mpe@...erman.id.au>
Cc: Geert Uytterhoeven <geert@...ux-m68k.org>,
	Christophe Leroy <christophe.leroy@...roup.eu>,
	Arnd Bergmann <arnd@...db.de>,
	Richard Henderson <richard.henderson@...aro.org>,
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Subject: Re: [PATCH RFC v3 1/2] mm: Add personality flag to limit address to
 47 bits

On Wed, Sep 11, 2024 at 11:38:55PM +1000, Michael Ellerman wrote:
> Geert Uytterhoeven <geert@...ux-m68k.org> writes:
> > Hi Christophe,
> >
> > On Tue, Sep 10, 2024 at 11:21 AM Christophe Leroy
> > <christophe.leroy@...roup.eu> wrote:
> >> >>> diff --git a/include/uapi/linux/personality.h b/include/uapi/linux/personality.h
> >> >>> index 49796b7756af..cd3b8c154d9b 100644
> >> >>> --- a/include/uapi/linux/personality.h
> >> >>> +++ b/include/uapi/linux/personality.h
> >> >>> @@ -22,6 +22,7 @@ enum {
> >> >>>     WHOLE_SECONDS =         0x2000000,
> >> >>>     STICKY_TIMEOUTS =       0x4000000,
> >> >>>     ADDR_LIMIT_3GB =        0x8000000,
> >> >>> +   ADDR_LIMIT_47BIT =      0x10000000,
> >> >>>   };
> >> >>
> >> >> I wonder if ADDR_LIMIT_128T would be clearer?
> >> >>
> >> >
> >> > I don't follow, what does 128T represent?
> >>
> >> 128T is 128 Terabytes, that's the maximum size achievable with a 47BIT
> >> address, that naming would be more consistant with the ADDR_LIMIT_3GB
> >> just above that means a 3 Gigabytes limit.
> >
> > Hence ADDR_LIMIT_128TB?
> 
> Yes it should be 128TB. Typo by me.
> 
> cheers

47BIT was selected because the usecase for this flag is for applications
that want to store data in the upper bits of a virtual address space. In
this case, how large the virtual address space is irrelevant, and only
the number of bits that are being used, and hence the number of bits
that are free.

- Charlie