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[203.219.139.206]) by smtp.gmail.com with ESMTPSA id n6sm6227193pfa.28.2021.12.09.02.15.47 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 09 Dec 2021 02:15:48 -0800 (PST) Date: Thu, 09 Dec 2021 20:15:42 +1000 From: Nicholas Piggin Subject: Re: [PATCH v4 09/10] powerpc/mm: Convert to default topdown mmap layout To: "alex@ghiti.fr" , Benjamin Herrenschmidt , Christophe Leroy , Michael Ellerman , Paul Mackerras References: In-Reply-To: MIME-Version: 1.0 Message-Id: <1639044621.jeow25j0pr.astroid@bobo.none> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: quoted-printable X-BeenThere: linuxppc-dev@lists.ozlabs.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: "linux-mm@kvack.org" , "akpm@linux-foundation.org" , "linuxppc-dev@lists.ozlabs.org" , "linux-kernel@vger.kernel.org" Errors-To: linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@lists.ozlabs.org Sender: "Linuxppc-dev" Excerpts from Christophe Leroy's message of December 9, 2021 3:18 am: > Select CONFIG_ARCH_WANT_DEFAULT_TOPDOWN_MMAP_LAYOUT and > remove arch/powerpc/mm/mmap.c >=20 > This change provides standard randomisation of mmaps. >=20 > See commit 8b8addf891de ("x86/mm/32: Enable full randomization on i386 > and X86_32") for all the benefits of mmap randomisation. The justification seems pretty reasonable. >=20 > Comparison between powerpc implementation and the generic one: > - mmap_is_legacy() is identical. > - arch_mmap_rnd() does exactly the same allthough it's written > slightly differently. > - MIN_GAP and MAX_GAP are identical. > - mmap_base() does the same but uses STACK_RND_MASK which provides > the same values as stack_maxrandom_size(). > - arch_pick_mmap_layout() is almost identical. The only difference > is that it also adds the random factor to mm->mmap_base in legacy mode. >=20 > That last point is what provides the standard randomisation of mmaps. Thanks for describing it. Could you add random_factor to mmap_base for the legacy path for powerpc as a 2-line change that adds the legacy randomisation. And then this bigger patch would be closer to a no-op. Thanks, Nick