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

From: Charlie Jenkins <charlie@rivosinc.com>
To: Arnd Bergmann <arnd@arndb.de>
Cc: "Liam R. Howlett" <Liam.Howlett@oracle.com>,
	Catalin Marinas <catalin.marinas@arm.com>,
	guoren <guoren@kernel.org>,
	Richard Henderson <richard.henderson@linaro.org>,
	Ivan Kokshaysky <ink@jurassic.park.msu.ru>,
	Matt Turner <mattst88@gmail.com>,
	Vineet Gupta <vgupta@kernel.org>,
	Russell King <linux@armlinux.org.uk>,
	Huacai Chen <chenhuacai@kernel.org>,
	WANG Xuerui <kernel@xen0n.name>,
	Thomas Bogendoerfer <tsbogend@alpha.franken.de>,
	"James E . J . Bottomley" <James.Bottomley@hansenpartnership.com>,
	Helge Deller <deller@gmx.de>,
	Michael Ellerman <mpe@ellerman.id.au>,
	Nicholas Piggin <npiggin@gmail.com>,
	Christophe Leroy <christophe.leroy@csgroup.eu>,
	Naveen N Rao <naveen@kernel.org>,
	Alexander Gordeev <agordeev@linux.ibm.com>,
	Gerald Schaefer <gerald.schaefer@linux.ibm.com>,
	Heiko Carstens <hca@linux.ibm.com>,
	Vasily Gorbik <gor@linux.ibm.com>,
	Christian Borntraeger <borntraeger@linux.ibm.com>,
	Sven Schnelle <svens@linux.ibm.com>,
	Yoshinori Sato <ysato@users.sourceforge.jp>,
	Rich Felker <dalias@libc.org>,
	John Paul Adrian Glaubitz <glaubitz@physik.fu-berlin.de>,
	"David S . Miller" <davem@davemloft.net>,
	Andreas Larsson <andreas@gaisler.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	x86@kernel.org, "H. Peter Anvin" <hpa@zytor.com>,
	Andy Lutomirski <luto@kernel.org>,
	Peter Zijlstra <peterz@infradead.org>,
	Muchun Song <muchun.song@linux.dev>,
	Andrew Morton <akpm@linux-foundation.org>,
	Vlastimil Babka <vbabka@suse.cz>,
	Lorenzo Stoakes <lorenzo.stoakes@oracle.com>,
	shuah <shuah@kernel.org>, Christoph Hellwig <hch@infradead.org>,
	Michal Hocko <mhocko@suse.com>,
	"Kirill A. Shutemov" <kirill@shutemov.name>,
	Chris Torek <chris.torek@gmail.com>,
	Linux-Arch <linux-arch@vger.kernel.org>,
	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" <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
Date: Wed, 11 Sep 2024 23:06:46 -0700	[thread overview]
Message-ID: <ZuKE9nnNGR/5fHh/@ghost> (raw)
In-Reply-To: <8130e50c-01e2-45c3-a516-45f5499311f2@app.fastmail.com>

On Wed, Sep 11, 2024 at 07:25:08AM +0000, Arnd Bergmann wrote:
> On Wed, Sep 11, 2024, at 00:45, Charlie Jenkins wrote:
> > On Tue, Sep 10, 2024 at 03:08:14PM -0400, Liam R. Howlett wrote:
> >
> > 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 other issue I have with this is that if there is not a hint address
> > specified to be greater than 47 bits on x86, then mmap() may return an
> > address that is greater than 47-bits. The documentation in
> > Documentation/arch/x86/x86_64/5level-paging.rst says:
> >
> > "If hint address set above 47-bit, but MAP_FIXED is not specified, we try
> > to look for unmapped area by specified address. If it's already
> > occupied, we look for unmapped area in *full* address space, rather than
> > from 47-bit window."
> 
> This is also in the commit message of b569bab78d8d ("x86/mm: Prepare
> to expose larger address space to userspace"), which introduced it.
> However, I don't actually see the fallback to the full address space,
> instead the actual behavior seems to be the same as arm64.
> 
> Am I missing something in the x86 implementation, or do we just
> need to update the documentation?
> 
>       Arnd

Yeah I guess it is incorrect documentation then? It seems more
reasonable to me to have a hint address fall back onto the larger
address space because otherwise the "hint" address can cause allocations
to fail even if there is space above the 47-bit limit. This is another
reason I wanted to avoid having this default behavior on riscv, to not
have this abuse of the hint address.

- Charlie