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Shutemov" , x86@kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <20240307133916.3782068-1-yosryahmed@google.com> <20240307133916.3782068-3-yosryahmed@google.com> <83b019e2-1b84-491a-b0b9-beb02e45d80c@intel.com> From: Dave Hansen 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: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-Rspamd-Server: rspam08 X-Rspamd-Queue-Id: ACD43A0009 X-Stat-Signature: qbp6dkmsmqw5pgdq5ei9ef5i8a9t5w1s X-Rspam-User: X-HE-Tag: 1709847595-70928 X-HE-Meta: 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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 3/7/24 13:04, Yosry Ahmed wrote: > I thought about doing inc_mm_tlb_gen() when LAM is updated, but it felt > hacky and more importantly doesn't make it clear in switch_mm_irqs_off() > that we correctly handle LAM updates. We can certainly add a comment, > but I think an explicit check for CPU LAM vs. mm LAM is much clearer. > > WDYT? The mm generations are literally there so that if the mm changes that all the CPUs know they need an update. Changing LAM enabling is 100% consistent with telling other CPUs that they need an update. I'd be curious of Andy feels differently though. >> Considering how fun this code path is, a little effort at an actual >> reproduction would be really appreciated. > > I tried reproducing it but gave up quickly. We need a certain sequence > of events to happen: > > CPU 1 CPU 2 > kthread_use_mm() > /* user thread enables LAM */ > context_switch() > context_switch() /* to user thread */ First, it would be fine to either create a new kthread for reproduction purposes or to hack an existing one. For instance, have have the LAM prctl() take an extra ref on the mm and stick it in a global variable: mmgrab(current->mm); global_mm = current->mm; Then in the kthread, grab the mm and use it: while (!global_mm); kthread_use_mm(global_mm); ... check for the race mmdrop(global_mm); You can also hackily wait for thread to move with a stupid spin loop: while (smp_processor_id() != 1); and then actually move it with sched_setaffinity() from userspace. That can make it easier to get that series of events to happen in lockstep.