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J . Bottomley" , Helge Deller , Michael Ellerman , Nicholas Piggin , Christophe Leroy , Naveen N Rao , Alexander Gordeev , Gerald Schaefer , Heiko Carstens , Vasily Gorbik , Christian Borntraeger , Sven Schnelle , Yoshinori Sato , Rich Felker , John Paul Adrian Glaubitz , "David S . Miller" , Andreas Larsson , Thomas Gleixner , Ingo Molnar , Borislav Petkov , Dave Hansen , x86@kernel.org, "H. Peter Anvin" , Andy Lutomirski , Peter Zijlstra , Muchun Song , Andrew Morton , "Liam R. Howlett" , Vlastimil Babka , shuah , Christoph Hellwig , Michal Hocko , "Kirill A. Shutemov" , Chris Torek , Linux-Arch , linux-kernel@vger.kernel.org, linux-alpha@vger.kernel.org, linux-snps-arc@lists.infradead.org, linux-arm-kernel@lists.infradead.org, "linux-csky@vger.kernel.org" , loongarch@lists.linux.dev, linux-mips@vger.kernel.org, linux-parisc@vger.kernel.org, linuxppc-dev@lists.ozlabs.org, linux-s390@vger.kernel.org, linux-sh@vger.kernel.org, sparclinux@vger.kernel.org, linux-mm@kvack.org, linux-kselftest@vger.kernel.org, linux-abi-devel@lists.sourceforge.net Subject: Re: [PATCH RFC v3 1/2] mm: Add personality flag to limit address to 47 bits Message-ID: <58f39d58-579e-4dd3-8084-baebf86f1ae0@lucifer.local> References: <20240905-patches-below_hint_mmap-v3-0-3cd5564efbbb@rivosinc.com> <20240905-patches-below_hint_mmap-v3-1-3cd5564efbbb@rivosinc.com> <9fc4746b-8e9d-4a75-b966-e0906187e6b7@app.fastmail.com> Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <9fc4746b-8e9d-4a75-b966-e0906187e6b7@app.fastmail.com> X-ClientProxiedBy: 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 bhATFoOe 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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: 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 > > 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