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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 X-Stat-Signature: jomnqj9jrj36fsbataehyg4k6ob9c58b X-Rspam-User: X-Rspamd-Server: rspam12 X-Rspamd-Queue-Id: ECA8A8000A X-HE-Tag: 1781828947-703979 X-HE-Meta: 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 5IFzlmpI uokydfKE41zINesFf4UZAHWR/49ALW3sgZjzDIhboDYOwZJd7ypClY6a+ji5JM1P/f9H5NfWGrI9+5Jcnx2rrY5EeAhxjKoQDurYquMbKrIvI2h8pPbLxn9+pHLsukLQzqKp5xIMLVKLln8z3ujY6W/+3FLPrWJ92ZG9e4TH22flJz//Q0zetFvRSM/ol2WBvu8COCCz9Uln7708r6vGAAjwrcc/jl2DE/Tk9NFlsYXe1fOeO2aTzo/kiCiInZHeqzL8XGi/gnpXRBPk2V2gz0E+JDA== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: 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?