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Peter Anvin" , linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, linux-riscv@lists.infradead.org, linux-mm@kvack.org Subject: Re: [PATCH 5/5] mm: Do not map the shadow stack as THP Message-ID: References: <20260224175800.2500729-1-catalin.marinas@arm.com> <20260224175800.2500729-6-catalin.marinas@arm.com> <5570c06d-fbc1-42c9-9b27-26211e8b033c@sirena.org.uk> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <5570c06d-fbc1-42c9-9b27-26211e8b033c@sirena.org.uk> X-Rspam-User: X-Rspamd-Server: rspam07 X-Rspamd-Queue-Id: E5E50100015 X-Stat-Signature: aj7je7y61s4ssmnn4mps9rrwbhqgdm6q X-HE-Tag: 1772034686-486307 X-HE-Meta: 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 eqpDZ1HA iatNYCUP7MV+faUjRWX4l3hVSm/gpvTrdKZqFHT5L3Lx5xTwiHMPsJEXwhY9WIIw7zRf1UtOstyNlsfBz2mhdHeCvG4OGv8/AaKJ+yZCZJn3gUnBHJJHOBsCs5H+e2WskPXPhJNAFdxhiT1+5NKsBwSLzZygnrU/vG/Dy Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Wed, Feb 25, 2026 at 01:02:36PM +0000, Mark Brown wrote: > On Tue, Feb 24, 2026 at 05:57:57PM +0000, Catalin Marinas wrote: > > The default shadow stack size allocated on first prctl() for the main > > thread or subsequently on clone() is either half of RLIMIT_STACK or half > > of a thread's stack size (for arm64). Both of these are likely to be > > suitable for a THP allocation and the kernel is more aggressive in > > creating such mappings. However, it does not make much sense to use a > > huge page. It didn't make sense for the normal stacks either, see commit > > c4608d1bf7c6 ("mm: mmap: map MAP_STACK to VM_NOHUGEPAGE"). > > Reviewed-by: Mark Brown Thanks. > The create THP and immediately splitting it pattern is very clear when > checking the mm behaviour on new GCSs, this should help performance. If the first access is a write, the kernel allocates a THP from the start without subsequent splitting. Also since 6.13 (commit 1ced09e0331f "mm: allocate THP on hugezeropage wp-fault"), we go for another THP on write. It's still wasting memory and time to zero the full 2MB when it's highly unlikely we'd ever use that much for a shadow stack. -- Catalin