From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mgamail.intel.com (mgamail.intel.com [198.175.65.20]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 483C87081E for ; Fri, 19 Jun 2026 00:29:07 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=198.175.65.20 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1781828949; cv=none; b=rUzp7SaMSEHXEr5P3KpRmLNDJLUi7+6dNkCFuvtl/zodHBEMbLeh+0/Bv40q0yDztIulYkZeyg8fBWmlLFxaK7f2f6goU+HwdKvH9zI7HSs49xaxq0aPoFtaOdyHJ+hrYFkriSOiavSwbVPDvZpjaCITO/IXPxQHDxFeRPT99T8= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1781828949; c=relaxed/simple; bh=WPmSPul8vCxi2V5AhWvJO1ewlmEn7LMsWjJ8y9duDmI=; h=Message-ID:Date:MIME-Version:From:Subject:To:Cc:References: In-Reply-To:Content-Type; b=bUJ71Rx/7juf1C4Uhpy54e1F67KoZ6R6ThCWjSi5vblhZ+GbhiVMe6omXZ92Dy8tdFwnjh21SaXVz/3pyK1bw7vKrtBR6rBpvPYlEUbxuFX7+BktirArepNacac2j8IvM4t+/pVhVJi/iGTQouZ63uT/lsQWKWDqinyXwPIP+94= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=intel.com; spf=pass smtp.mailfrom=intel.com; dkim=pass (2048-bit key) header.d=intel.com header.i=@intel.com header.b=evrMALae; arc=none smtp.client-ip=198.175.65.20 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=intel.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=intel.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=intel.com header.i=@intel.com header.b="evrMALae" DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=intel.com; i=@intel.com; q=dns/txt; s=Intel; t=1781828947; x=1813364947; h=message-id:date:mime-version:from:subject:to:cc: references:in-reply-to:content-transfer-encoding; bh=WPmSPul8vCxi2V5AhWvJO1ewlmEn7LMsWjJ8y9duDmI=; b=evrMALaew8V/1X7gcKQYeiZJw7DIU9U7WnWHZMqJ9vZwBVz1L90tS1Lg LYYtkp2rk25mlO3teC13QMHk/Pil1hjpoY0xbSbFOejuZSTbezBOW/d+q qXPygstAjxOGqoB9L/n5TxhpLBVARkzUDd2oo8MzxnLO0ZeHVqZ6DWVo6 bB+IfY4ZHfbjwxRT6aSBZjXrrLKstW3bCX2LzjzQNrkFkRFHWCIWQ4ioP 7nF3g6fsbh3YYbd+72TJxxjKzqkEPOg/x3DI8LGO5GApDjaKXlH0Fyba7 uiW1nOS7euhvFnXyEx5FEwPKj+/t2jrYy/rkgYzGZaCfJjs+C961eKcXM g==; X-CSE-ConnectionGUID: UQKlFKjWTwqSUPqKoNyo0Q== X-CSE-MsgGUID: bBIegkpGQ76cvEDrIEv+xg== X-IronPort-AV: E=McAfee;i="6800,10657,11821"; a="82458659" X-IronPort-AV: E=Sophos;i="6.24,212,1774335600"; d="scan'208";a="82458659" Received: from orviesa004.jf.intel.com ([10.64.159.144]) by orvoesa112.jf.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 18 Jun 2026 17:29:07 -0700 X-CSE-ConnectionGUID: Tktuk9LrStCg2JOj3fC/rQ== X-CSE-MsgGUID: dN2KD4aVRQKuxH2QPkka4Q== X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="6.24,212,1774335600"; d="scan'208";a="252828741" Received: from rfrazer-mobl3.amr.corp.intel.com (HELO [10.125.111.222]) ([10.125.111.222]) by orviesa004-auth.jf.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 18 Jun 2026 17:29:06 -0700 Message-ID: <85f19b94-18c6-471d-88f5-c05a2f10199b@intel.com> Date: Thu, 18 Jun 2026 17:29:05 -0700 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird From: Dave Hansen Subject: Re: [PATCH v2 00/13] Dynamic Kernel Stacks To: David Laight , Pasha Tatashin Cc: David Stevens , Linus Walleij , Will Deacon , Quentin Perret , Thomas Gleixner , Ingo Molnar , Borislav Petkov , Dave Hansen , x86@kernel.org, "H. Peter Anvin" , 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 References: <20260424191456.2679717-1-stevensd@google.com> <20260424232637.054f15dd@pumpkin> Content-Language: en-US Autocrypt: addr=dave.hansen@intel.com; keydata= xsFNBE6HMP0BEADIMA3XYkQfF3dwHlj58Yjsc4E5y5G67cfbt8dvaUq2fx1lR0K9h1bOI6fC oAiUXvGAOxPDsB/P6UEOISPpLl5IuYsSwAeZGkdQ5g6m1xq7AlDJQZddhr/1DC/nMVa/2BoY 2UnKuZuSBu7lgOE193+7Uks3416N2hTkyKUSNkduyoZ9F5twiBhxPJwPtn/wnch6n5RsoXsb ygOEDxLEsSk/7eyFycjE+btUtAWZtx+HseyaGfqkZK0Z9bT1lsaHecmB203xShwCPT49Blxz VOab8668QpaEOdLGhtvrVYVK7x4skyT3nGWcgDCl5/Vp3TWA4K+IofwvXzX2ON/Mj7aQwf5W iC+3nWC7q0uxKwwsddJ0Nu+dpA/UORQWa1NiAftEoSpk5+nUUi0WE+5DRm0H+TXKBWMGNCFn c6+EKg5zQaa8KqymHcOrSXNPmzJuXvDQ8uj2J8XuzCZfK4uy1+YdIr0yyEMI7mdh4KX50LO1 pmowEqDh7dLShTOif/7UtQYrzYq9cPnjU2ZW4qd5Qz2joSGTG9eCXLz5PRe5SqHxv6ljk8mb ApNuY7bOXO/A7T2j5RwXIlcmssqIjBcxsRRoIbpCwWWGjkYjzYCjgsNFL6rt4OL11OUF37wL QcTl7fbCGv53KfKPdYD5hcbguLKi/aCccJK18ZwNjFhqr4MliQARAQABzUVEYXZpZCBDaHJp c3RvcGhlciBIYW5zZW4gKEludGVsIFdvcmsgQWRkcmVzcykgPGRhdmUuaGFuc2VuQGludGVs LmNvbT7CwXgEEwECACIFAlQ+9J0CGwMGCwkIBwMCBhUIAgkKCwQWAgMBAh4BAheAAAoJEGg1 lTBwyZKwLZUP/0dnbhDc229u2u6WtK1s1cSd9WsflGXGagkR6liJ4um3XCfYWDHvIdkHYC1t MNcVHFBwmQkawxsYvgO8kXT3SaFZe4ISfB4K4CL2qp4JO+nJdlFUbZI7cz/Td9z8nHjMcWYF IQuTsWOLs/LBMTs+ANumibtw6UkiGVD3dfHJAOPNApjVr+M0P/lVmTeP8w0uVcd2syiaU5jB aht9CYATn+ytFGWZnBEEQFnqcibIaOrmoBLu2b3fKJEd8Jp7NHDSIdrvrMjYynmc6sZKUqH2 I1qOevaa8jUg7wlLJAWGfIqnu85kkqrVOkbNbk4TPub7VOqA6qG5GCNEIv6ZY7HLYd/vAkVY E8Plzq/NwLAuOWxvGrOl7OPuwVeR4hBDfcrNb990MFPpjGgACzAZyjdmYoMu8j3/MAEW4P0z F5+EYJAOZ+z212y1pchNNauehORXgjrNKsZwxwKpPY9qb84E3O9KYpwfATsqOoQ6tTgr+1BR CCwP712H+E9U5HJ0iibN/CDZFVPL1bRerHziuwuQuvE0qWg0+0SChFe9oq0KAwEkVs6ZDMB2 P16MieEEQ6StQRlvy2YBv80L1TMl3T90Bo1UUn6ARXEpcbFE0/aORH/jEXcRteb+vuik5UGY 5TsyLYdPur3TXm7XDBdmmyQVJjnJKYK9AQxj95KlXLVO38lczsFNBFRjzmoBEACyAxbvUEhd GDGNg0JhDdezyTdN8C9BFsdxyTLnSH31NRiyp1QtuxvcqGZjb2trDVuCbIzRrgMZLVgo3upr MIOx1CXEgmn23Zhh0EpdVHM8IKx9Z7V0r+rrpRWFE8/wQZngKYVi49PGoZj50ZEifEJ5qn/H Nsp2+Y+bTUjDdgWMATg9DiFMyv8fvoqgNsNyrrZTnSgoLzdxr89FGHZCoSoAK8gfgFHuO54B lI8QOfPDG9WDPJ66HCodjTlBEr/Cwq6GruxS5i2Y33YVqxvFvDa1tUtl+iJ2SWKS9kCai2DR 3BwVONJEYSDQaven/EHMlY1q8Vln3lGPsS11vSUK3QcNJjmrgYxH5KsVsf6PNRj9mp8Z1kIG qjRx08+nnyStWC0gZH6NrYyS9rpqH3j+hA2WcI7De51L4Rv9pFwzp161mvtc6eC/GxaiUGuH BNAVP0PY0fqvIC68p3rLIAW3f97uv4ce2RSQ7LbsPsimOeCo/5vgS6YQsj83E+AipPr09Caj 0hloj+hFoqiticNpmsxdWKoOsV0PftcQvBCCYuhKbZV9s5hjt9qn8CE86A5g5KqDf83Fxqm/ vXKgHNFHE5zgXGZnrmaf6resQzbvJHO0Fb0CcIohzrpPaL3YepcLDoCCgElGMGQjdCcSQ+Ci FCRl0Bvyj1YZUql+ZkptgGjikQARAQABwsFfBBgBAgAJBQJUY85qAhsMAAoJEGg1lTBwyZKw l4IQAIKHs/9po4spZDFyfDjunimEhVHqlUt7ggR1Hsl/tkvTSze8pI1P6dGp2XW6AnH1iayn yRcoyT0ZJ+Zmm4xAH1zqKjWplzqdb/dO28qk0bPso8+1oPO8oDhLm1+tY+cOvufXkBTm+whm +AyNTjaCRt6aSMnA/QHVGSJ8grrTJCoACVNhnXg/R0g90g8iV8Q+IBZyDkG0tBThaDdw1B2l asInUTeb9EiVfL/Zjdg5VWiF9LL7iS+9hTeVdR09vThQ/DhVbCNxVk+DtyBHsjOKifrVsYep WpRGBIAu3bK8eXtyvrw1igWTNs2wazJ71+0z2jMzbclKAyRHKU9JdN6Hkkgr2nPb561yjcB8 sIq1pFXKyO+nKy6SZYxOvHxCcjk2fkw6UmPU6/j/nQlj2lfOAgNVKuDLothIxzi8pndB8Jju KktE5HJqUUMXePkAYIxEQ0mMc8Po7tuXdejgPMwgP7x65xtfEqI0RuzbUioFltsp1jUaRwQZ MTsCeQDdjpgHsj+P2ZDeEKCbma4m6Ez/YWs4+zDm1X8uZDkZcfQlD9NldbKDJEXLIjYWo1PH hYepSffIWPyvBMBTW2W5FRjJ4vLRrJSUoEfJuPQ3vW9Y73foyo/qFoURHO48AinGPZ7PC7TF vUaNOTjKedrqHkaOcqB185ahG2had0xnFsDPlx5y In-Reply-To: <20260424232637.054f15dd@pumpkin> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 4/24/26 15:26, David Laight wrote: >> This true until, in a fleet of millions of machines, you encounter a >> one-in-a-billion chance of a stack overflow. You are then forced to >> double the statically allocated kernel stacks on every machine, paying a >> memory tax even though 99.999..% of threads never exceed 4K. This >> overhead accumulates to petabytes of wasted capacity. > And then you hit a stack fault in some path where you can't sleep and > there isn't any available kernel memory. > > An alternative idea is to arrange for some system calls to sleep in > userspace, so when the thread is woken it re-executes the system call. > It then makes sense to assign the kernel stack to the process when > it enters the kernel. There are probably other ways to do this without handling exceptions. For instance, let's say you always *map* 16k of stack for each process. But, after context switching out, you take a look at 4x8b pte_t's that were mapping the kernel stack. If the _PAGE_ACCESSED bit is clear, you can just clear _PAGE_PRESENT and reclaim the page. If you don't want the overhead in the normal context switch path, you reclaim in a shrinker, at the cost of needing locking to coordinate with the scheduler. A simple rule would be: a thread that ever accesses a page gets to keep it forever. They're never reclaimed after being accessed, only before. For that, the worst case is that you go to schedule a new thread and can't allocate memory fill in the 4 pte_t's. You can't run it until you or some other CPU goes and does some reclaim. Needing memory in the middle of schedule() is generally a no-go. But its a lot better than not being able to continue _execution_ of a kernel thread at *ALL*, possibly in a non-preemptible context, like when you do it in a #PF. Basically, I think there's a way to do this that limits the kernel blast radius to _mostly_ being a core mm problem. What else has been considered before the #PF-based mechanism?