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[92.21.50.228]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-4756636cde2sm798992f8f.20.2026.06.29.13.28.23 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 29 Jun 2026 13:28:24 -0700 (PDT) Date: Mon, 29 Jun 2026 21:28:22 +0100 From: David Laight To: Dave Hansen Cc: Thomas Gleixner , Zach O'Keefe , "H. Peter Anvin" , David Stevens , Pasha Tatashin , Linus Walleij , Will Deacon , Quentin Perret , Ingo Molnar , Borislav Petkov , Dave Hansen , x86@kernel.org, Andy Lutomirski , Xin Li , Peter Zijlstra , Andrew Morton , David Hildenbrand , Lorenzo Stoakes , "Liam R. Howlett" , Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , Uladzislau Rezki , Kees Cook , linux-kernel@vger.kernel.org, linux-mm@kvack.org Subject: Re: [PATCH v2 00/13] Dynamic Kernel Stacks Message-ID: <20260629212822.14dbb27f@pumpkin> In-Reply-To: <5604a47b-9457-4162-bd23-720e29cf1983@intel.com> References: <20260424191456.2679717-1-stevensd@google.com> <6369e5ce-74e3-4c68-8053-d7d7d21b6955@zytor.com> <87pl1md7h0.ffs@fw13> <87qzm2b39k.ffs@fw13> <87mrwon5uw.ffs@fw13> <87cxxgly12.ffs@fw13> <5604a47b-9457-4162-bd23-720e29cf1983@intel.com> X-Mailer: Claws Mail 4.1.1 (GTK 3.24.38; arm-unknown-linux-gnueabihf) MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: quoted-printable X-Rspamd-Server: rspam05 X-Rspamd-Queue-Id: 9B33B80003 X-Rspam-User: X-Stat-Signature: nhj6z55581zrx4hginxirwo519r8463g X-HE-Tag: 1782764906-727902 X-HE-Meta: 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 Olvk5K54 gUckqq4/gReBMaFCeVkrRDHPKJEs3WCy0vAUHrb00lEOQI8x27249IT3D237WIMENd1khF8J5tZ37XRS9v3wtoer2cICqqSvNT1X2pUBjbY3YIWA03gpKszNNGu/YkNRLEA5iZPYD9tVGvq2qATb+ypDBoz1PMjH/loXb4sRVAG1rOX2WSVUIWHT2M14bzXxKnw2gM501vQwaBjKZPNRTROLVkzcC4c/slQr1ONt82O4/trn6sQVqmNrSaidYWPhECUZIZIG5AMdwCHtyi5Ir2nQHK3P6qAmUL60FbHkYW9yv4CYTrJOorNjCBprXWuk225JbctoggsR79niXScGJMs1KB93IETYs/Tzz0YvgKWW/cf6w32YzOiuwvYCYxPAkypgZmCTfWGzGHQHahUENy8Dh4tF4NsxbBvd6CoUrdyqqDZCLpmivghZM4FZHtv/p5VuTEDFq9xLWSOo= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Mon, 29 Jun 2026 09:02:08 -0700 Dave Hansen wrote: > On 6/23/26 14:58, Thomas Gleixner wrote: > > So as you already confirmed that the number of offenders is really low, > > it'd be definitely a worthwhile effort to fix that for everyone and also > > make the compiler people to get their act together =F0=9F=99=82 =20 >=20 > One other thing comes to mind... >=20 > We have debugging options for looking at max stack depth, like > CONFIG_DEBUG_STACK_USAGE. But no distros seem to turn that on. There's > also some ftrace stuff, but I assume none of that is used at runtime eith= er. >=20 > Do we need some max stack depth instrumentation that folks will actually > turn on? It would be nice if we spat out warnings when folks went > through 12k (or 8k) of stack so there was a chance of closing the loop > and getting feedback from them. >=20 > It could be done with really little overhead if the vmalloc()'d stacks > set Accessed=3D0 on their PTEs and then checked them near vfree(). The > PTEs are already getting touched there, so the cachelines should be hot > anyway. For testing purposes you'd probably want to do the check in other places. Possibly slow paths that might be called with a deep stack as well as context switch and return to uses. I guess you could copy the stack (to global data) so that it could be dumped for diagnostics (finding the relevant stack trace is left as an exercise to the reader). Similarly it might be worth allocating an extra 4k page so that stack could be offset so the warning happens at (say) 12k. David >=20 > The granularity would only be 4k, but it would be so cheap that we could > turn it on universally. It would also be 100% deterministic. >=20 > There are, of course, more games that could be played with stack depth > checks on normal interrupts or NMIs. But those would be less deterministi= c. >=20 > Thoughts? >=20