From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 4AEFE19EEC8; Wed, 11 Sep 2024 18:21:39 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1726078900; cv=none; b=ADxxuzWT9kup5+yhfqr9bgy7WdBQdJg57MMADQ11vdPc8Sst1N2lMRHpBJcsn0+JQq55oD6sKqHCdtig5ePBU92NQwMjHv4Lpg9STS+e0grMY25piFwuZCXZYAnJ6ITAmRPKbwOtzguP6lI/QWpqNj1B42kYks1MTlPX61hc/TU= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1726078900; c=relaxed/simple; bh=2cZas4SimbOELK54ILhf2k9/V2LxmAALpH5ubeNMXEs=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=kP/hclgiLFbxRWj91GNSHxe6sYCCLMux96HnYWTDvZ9NcocJKq5MrCWckIlfIN0+rhF6hZS2dNmpTyqSvL0GYdB/cUnozR6jSj1UIKg5oqxS3nCZlLjqPEGY6+EDB0qqzgw+lUkRDPPDYV6pzJKLGTkQBD/HD3VtyscqUWQhlGI= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 Received: by smtp.kernel.org (Postfix) with ESMTPSA id 3AB7BC4CEC0; Wed, 11 Sep 2024 18:21:30 +0000 (UTC) Date: Wed, 11 Sep 2024 19:21:27 +0100 From: Catalin Marinas To: Charlie Jenkins Cc: "Liam R. Howlett" , Arnd Bergmann , guoren , Richard Henderson , Ivan Kokshaysky , Matt Turner , Vineet Gupta , Russell King , Huacai Chen , WANG Xuerui , Thomas Bogendoerfer , "James E . 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 , Vlastimil Babka , Lorenzo Stoakes , 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: 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> Precedence: bulk X-Mailing-List: linux-alpha@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: On Tue, Sep 10, 2024 at 05:45:07PM -0700, Charlie Jenkins wrote: > On Tue, Sep 10, 2024 at 03:08:14PM -0400, Liam R. Howlett wrote: > > * Catalin Marinas [240906 07:44]: > > > On Fri, Sep 06, 2024 at 09:55:42AM +0000, Arnd Bergmann wrote: > > > > On Fri, Sep 6, 2024, at 09:14, Guo Ren wrote: > > > > > On Fri, Sep 6, 2024 at 3:18 PM Arnd Bergmann wrote: > > > > >> 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. > > > > > > > > > > To optimize RISC-V progress, I recommend: > > > > > > > > > > Step 1: Approve the patch. > > > > > Step 2: Update Go and OpenJDK's RISC-V backend to utilize it. > > > > > Step 3: Wait approximately several iterations for Go & OpenJDK > > > > > Step 4: Remove the 47-bit constraint in arch_get_mmap_end() Point 4 is an ABI change. What guarantees that there isn't still software out there that relies on the old behaviour? > > > > I really want to first see a plausible explanation about why > > > > RISC-V can't just implement this using a 47-bit DEFAULT_MAP_WINDOW > > > > like all the other major architectures (x86, arm64, powerpc64), > > > > > > FWIW arm64 actually limits DEFAULT_MAP_WINDOW to 48-bit in the default > > > configuration. We end up with a 47-bit with 16K pages but for a > > > different reason that has to do with LPA2 support (I doubt we need this > > > for the user mapping but we need to untangle some of the macros there; > > > that's for a separate discussion). > > > > > > That said, we haven't encountered any user space problems with a 48-bit > > > DEFAULT_MAP_WINDOW. So I also think RISC-V should follow a similar > > > approach (47 or 48 bit default limit). Better to have some ABI > > > consistency between architectures. One can still ask for addresses above > > > this default limit via mmap(). > > > > I think that is best as well. > > > > Can we please just do what x86 and arm64 does? > > I responded to Arnd in the other thread, but I am still not convinced > that the solution that x86 and arm64 have selected is the best solution. > The solution of defaulting to 47 bits does allow applications the > ability to get addresses that are below 47 bits. However, due to > differences across architectures it doesn't seem possible to have all > architectures default to the same value. Additionally, this flag will be > able to help users avoid potential bugs where a hint address is passed > that causes upper bits of a VA to be used. The reason we added this limit on arm64 is that we noticed programs using the top 8 bits of a 64-bit pointer for additional information. IIRC, it wasn't even openJDK but some JavaScript JIT. We could have taught those programs of a new flag but since we couldn't tell how many are out there, it was the safest to default to a smaller limit and opt in to the higher one. Such opt-in is via mmap() but if you prefer a prctl() flag, that's fine by me as well (though I think this should be opt-in to higher addresses rather than opt-out of the higher addresses). -- Catalin