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[109.81.29.251]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-42cb7f2e598sm32381240f8f.4.2025.11.25.00.45.39 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 25 Nov 2025 00:45:39 -0800 (PST) Date: Tue, 25 Nov 2025 09:45:38 +0100 From: Michal Hocko To: Joshua Hahn Cc: Andrew Morton , Vlastimil Babka , Mike Rapoport , linux-kernel@vger.kernel.org, linux-mm@kvack.org, kernel-team@meta.com Subject: Re: [PATCH v2 2/2] mm/mm_init: decouple page checking and init_on_{alloc, free} Message-ID: References: <20251124225408.2243564-1-joshua.hahnjy@gmail.com> <20251124225408.2243564-2-joshua.hahnjy@gmail.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20251124225408.2243564-2-joshua.hahnjy@gmail.com> On Mon 24-11-25 14:54:07, Joshua Hahn wrote: > init_on_alloc and init_on_free protect the kernel by initializing > allocated and freed pages to 0 on allocation time / deletion. > Commit 700d2e9a36b93601270c1e15550acde2521386c5 ("mm, page_alloc: reduce > page alloc/free sanity checks") removed page checking from hot pcp > drain and refill paths, and instead coupled it with CONFIG_DEBUG_VM, > debug_pagealloc, page poisoning, and init_on_{alloc, free}. > > As the commit suggests, the first three turn the kernel into a debug > kernel, while the last hardens the kernel against leaking sensitive memory. > While enabling page checking is relatively low-cost and tying it > together with page initialization is not unreasonable, it does feel like > a bit of a side-effect, rather than an obvious consequence. > > With page checking now pulled out as a boot time parameter that can be > set independently, let's decouple page checking and init_on_alloc and > init_on_free. > > As a direct side effect, systems that have init_on_alloc or init_on_free > will no longer have page checking enabled by default; they will either > have to pass the check_pages boot parameter, build the kernel with > CONFIG_DEBUG_VM, or enable debug_pagealloc / page poisoning. How come this will not break existing users? What is an actual upside to get for the risk involved? -- Michal Hocko SUSE Labs