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Message-ID: <58f39d58-579e-4dd3-8084-baebf86f1ae0@lucifer.local>
Date: Fri, 6 Sep 2024 09:14:17 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: Arnd Bergmann <arnd@...db.de>
Cc: Charlie Jenkins <charlie@...osinc.com>,
        Richard Henderson <richard.henderson@...aro.org>,
        Ivan Kokshaysky <ink@...assic.park.msu.ru>,
        Matt Turner <mattst88@...il.com>, Vineet Gupta <vgupta@...nel.org>,
        Russell King <linux@...linux.org.uk>, guoren <guoren@...nel.org>,
        Huacai Chen <chenhuacai@...nel.org>, WANG Xuerui <kernel@...0n.name>,
        Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
        "James E . J . Bottomley" <James.Bottomley@...senpartnership.com>,
        Helge Deller <deller@....de>, Michael Ellerman <mpe@...erman.id.au>,
        Nicholas Piggin <npiggin@...il.com>,
        Christophe Leroy <christophe.leroy@...roup.eu>,
        Naveen N Rao <naveen@...nel.org>,
        Alexander Gordeev <agordeev@...ux.ibm.com>,
        Gerald Schaefer <gerald.schaefer@...ux.ibm.com>,
        Heiko Carstens <hca@...ux.ibm.com>, Vasily Gorbik <gor@...ux.ibm.com>,
        Christian Borntraeger <borntraeger@...ux.ibm.com>,
        Sven Schnelle <svens@...ux.ibm.com>,
        Yoshinori Sato <ysato@...rs.sourceforge.jp>,
        Rich Felker <dalias@...c.org>,
        John Paul Adrian Glaubitz <glaubitz@...sik.fu-berlin.de>,
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        Muchun Song <muchun.song@...ux.dev>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Liam R. Howlett" <Liam.Howlett@...cle.com>,
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        Christoph Hellwig <hch@...radead.org>, Michal Hocko <mhocko@...e.com>,
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Subject: Re: [PATCH RFC v3 1/2] mm: Add personality flag to limit address to
 47 bits

On Fri, Sep 06, 2024 at 07:17:44AM GMT, Arnd Bergmann wrote:
> On Thu, Sep 5, 2024, at 21:15, Charlie Jenkins wrote:
> > Create a personality flag ADDR_LIMIT_47BIT to support applications
> > that wish to transition from running in environments that support at
> > most 47-bit VAs to environments that support larger VAs. This
> > personality can be set to cause all allocations to be below the 47-bit
> > boundary. Using MAP_FIXED with mmap() will bypass this restriction.
> >
> > Signed-off-by: Charlie Jenkins <charlie@...osinc.com>
>
> I think having an architecture-independent mechanism to limit the size
> of the 64-bit address space is useful in general, and we've discussed
> the same thing for arm64 in the past, though we have not actually
> reached an agreement on the ABI previously.

The thread on the original proposals attests to this being rather a fraught
topic, and I think the weight of opinion was more so in favour of opt-in
rather than opt-out.

>
> > @@ -22,6 +22,7 @@ enum {
> >  	WHOLE_SECONDS =		0x2000000,
> >  	STICKY_TIMEOUTS	=	0x4000000,
> >  	ADDR_LIMIT_3GB = 	0x8000000,
> > +	ADDR_LIMIT_47BIT = 	0x10000000,
> > };
>
> I'm a bit worried about having this done specifically in the
> personality flag bits, as they are rather limited. We obviously
> don't want to add many more such flags when there could be
> a way to just set the default limit.

Since I'm the one who suggested it, I feel I should offer some kind of
vague defence here :)

We shouldn't let perfect be the enemy of the good. This is a relatively
straightforward means of achieving the aim (assuming your concern about
arch_get_mmap_end() below isn't a blocker) which has the least impact on
existing code.

Of course we can end up in absurdities where we start doing
ADDR_LIMIT_xxBIT... but again - it's simple, shouldn't represent an
egregious maintenance burden and is entirely opt-in so has things going for
it.

>
> It's also unclear to me how we want this flag to interact with
> the existing logic in arch_get_mmap_end(), which attempts to
> limit the default mapping to a 47-bit address space already.

How does ADDR_LIMIT_3GB presently interact with that?

>
> For some reason, it appears that the arch_get_mmap_end()
> logic on RISC-V defaults to the maximum address
> space for the 'addr==0' case which is inconsistentn with
> the other architectures, so we should probably fix that
> part first, possibly moving more of that logic into a
> shared implementation.
>
>       Arnd

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