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

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Message-ID: <87y13mnc57.fsf@mail.lhotse>
Date: Fri, 20 Sep 2024 15:10:28 +1000
From: Michael Ellerman <mpe@...erman.id.au>
To: Charlie Jenkins <charlie@...osinc.com>
Cc: Geert Uytterhoeven <geert@...ux-m68k.org>, Christophe Leroy
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Subject: Re: [PATCH RFC v3 1/2] mm: Add personality flag to limit address to
 47 bits

Charlie Jenkins <charlie@...osinc.com> writes:
> 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.
>
> 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.

Yeah I understand that's how you came to the problem.

But for the user API I think using the size of the address space is
clearer, easier to explain, and matches the existing ADDR_LIMIT_3GB.

cheers