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Shutemov" , Yosry Ahmed Cc: Andrew Morton , Thomas Gleixner , Ingo Molnar , Borislav Petkov , Peter Zijlstra , Andy Lutomirski , 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> 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-Rspam-User: X-Rspamd-Server: rspam06 X-Rspamd-Queue-Id: 3477612000F X-Stat-Signature: 1iji4osm5fjz3choqix5s3595ertzzzp X-HE-Tag: 1709834168-492126 X-HE-Meta: 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 3xRCHUYb AYkPlJ5K8GhUnTMVz2PAQG0brHnClyxAC0VN4SLw69M69ae6AVs0JMIJuRjlSVO72V1Lc0+8N7Fk2MYXj4ZDtWkeFFU4QTD1C3S8+0qJX2nJiNofb6aC1Gh39BknfLUixE/lYyZrov9E15PGe+OCWx7mxryTqxuZplSDaDgwETaYpv8qz/ASGvAxKqQ== 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 09:29, Kirill A. Shutemov wrote: > On Thu, Mar 07, 2024 at 01:39:15PM +0000, Yosry Ahmed wrote: >> During context switching, if we are not switching to new mm and no TLB >> flush is needed, we do not write CR3. However, it is possible that a >> user thread enables LAM while a kthread is running on a different CPU >> with the old LAM CR3 mask. If the kthread context switches into any >> thread of that user process, it may not write CR3 with the new LAM mask, >> which would cause the user thread to run with a misconfigured CR3 that >> disables LAM on the CPU. > I don't think it is possible. As I said we can only enable LAM when the > process has single thread. If it enables LAM concurrently with kernel > thread and kernel thread gets control on the same CPU after the userspace > thread of the same process LAM is already going to be enabled. No need in > special handling. I think it's something logically like this: // main thread kthread_use_mm() cr3 |= mm->lam_cr3_mask; mm->lam_cr3_mask = foo; cpu_tlbstate.lam = mm->lam_cr3_mask; Obviously the kthread's LAM state is going to be random. It's fundamentally racing with the enabling thread. That part is fine. The main pickle is the fact that CR3 and cpu_tlbstate.lam are out of sync. That seems worth fixing. Or is there something else that keeps this whole thing from racing in the first place?