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X-CSE-ConnectionGUID: vp4B2bNQRbacE6azgULS+A== X-CSE-MsgGUID: KCxZa1tjRK2ZESEH771Eow== X-IronPort-AV: E=McAfee;i="6600,9927,11087"; a="13372377" X-IronPort-AV: E=Sophos;i="6.08,198,1712646000"; d="scan'208";a="13372377" Received: from fmviesa009.fm.intel.com ([10.60.135.149]) by orvoesa113.jf.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 29 May 2024 09:41:25 -0700 X-CSE-ConnectionGUID: G80LEDOmSAylmCLCHSRfGg== X-CSE-MsgGUID: 4cIPuAd3RueYH9hdPiHLSA== X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="6.08,198,1712646000"; d="scan'208";a="35577994" Received: from kinlongk-mobl1.amr.corp.intel.com (HELO [10.125.111.96]) ([10.125.111.96]) by fmviesa009-auth.fm.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 29 May 2024 09:41:24 -0700 Message-ID: <961f9533-1e0c-416c-b6b0-d46b97127de2@intel.com> Date: Wed, 29 May 2024 09:41:22 -0700 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v10 00/12] LUF(Lazy Unmap Flush) reducing tlb numbers over 90% To: Byungchul Park Cc: "Huang, Ying" , linux-kernel@vger.kernel.org, linux-mm@kvack.org, kernel_team@skhynix.com, akpm@linux-foundation.org, vernhao@tencent.com, mgorman@techsingularity.net, hughd@google.com, willy@infradead.org, david@redhat.com, peterz@infradead.org, luto@kernel.org, tglx@linutronix.de, mingo@redhat.com, bp@alien8.de, dave.hansen@linux.intel.com, rjgolo@gmail.com References: <20240510065206.76078-1-byungchul@sk.com> <982317c0-7faa-45f0-82a1-29978c3c9f4d@intel.com> <20240527015732.GA61604@system.software.com> <8734q46jc8.fsf@yhuang6-desk2.ccr.corp.intel.com> <44e4f2fd-e76e-445d-b618-17a6ec692812@intel.com> <20240529050046.GB20307@system.software.com> From: Dave Hansen Content-Language: en-US Autocrypt: addr=dave.hansen@intel.com; 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charset=UTF-8 Content-Transfer-Encoding: 7bit X-Stat-Signature: 8a7c67mzztte7xm46w995y9gwimyiw3m X-Rspamd-Queue-Id: B0F4B20013 X-Rspam-User: X-Rspamd-Server: rspam10 X-HE-Tag: 1717000886-206332 X-HE-Meta: U2FsdGVkX19W7VZIFr91LUc0BrIW1OJsoesTY5i1eeHTNztnIalyNuYWZE04fi6Ktea3xKQ+iOdXF4hrN+qhLAw9qHxYcijzHVSanXXWpK0GPSkmVbo3Oga+fF1gYzP/g7vS3afZTEbuWnUVGy/rGebfcX/LYAKB8tXpgZafT0iZHVXVdq5u+8dMVp16JuQE72ePu4QPNNptJB0M4BvtljCg/YOhcf7GtjVut5vfMP4UwaGhwsDTUx6JXqLepGb9H7bWdzfSGuNSEIfgA8vAzosFvabeyXycWnIzCBPQBdVVqHIcukNC2+ZXzjDr+zwVVflZ0Okj1tBV8IgO2P/ieNAer0Mnyx9Iw9iY9tBirHaPlDFuV1zxJrnHyjpLOTuJm/7wBo6AJbRnuc7DEVH7KDDShmDXJtAsai6a3gZpooXccbxkslXbTqZjuT7cXLHffOBxpReOScNdsosjCSyDJKvxdq1iN2ZvJ0rMwdBcJXJOR9hYc75hbyt8C5HB2gEGkwLq8BdSbMCubljB+x1eikJfLgeO2FQph2h3DJzSP5fhfZmTK2+QnbUeqS331256zPir6ZH7dWo5JA7s5uM4Aby76bXrWgazVwXToI3vmn0vsRa51QZUwAjAiZphpSuCgM3gLSZFiLOg/GVs117zXwcy1XvF4vDiHNOjAewnpJ6xmmzsjx5BS0WS4Al16NVxeIqXSCpgMN9chXQ4UPrJjANRnmvYmMObtRbp2f7anIrGYh0mySHxc2OkQQUdkSxZp2QQGkuxW0X+Ymqv3vSGemTHXAuX9e4HRdJS8WMvURLH/g9soH5YjWa6Y5ObAcqLqzePy7lnQycujni1q2RenWxhCzSoUuwgePl/5uY/+JThLC45jJMsWEgVGzQPzhw+TrjkM7GocwvaiGDjf/ZmSE1N1WbD6yXAnf0NMDmQeCdb14HK1AdgP58rGBHsaRZhJl+1mhxzmQTbfjGb82K Gw5Csp+t ygohhyyG10sdwdMj6VFSNep7gpqb/knz+Mur+rgzKDuLUcH0NZWoGDZ/OS9MPjxrkGmcWF8b6959Kmw2Pl4qwu1bNQaq+HMwFBGaDkG9I5kAMS6GNtXh8vPNWqApDp6dwd5t9m4zZRujxVMPo9jYaIeOOoBr8cNovexnlMvBtwAZyl+nXVywaJIZF6A== 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 5/28/24 22:00, Byungchul Park wrote: > All the code updating ptes already performs TLB flush needed in a safe > way if it's inevitable e.g. munmap. LUF which controls when to flush in > a higer level than arch code, just leaves stale ro tlb entries that are > currently supposed to be in use. Could you give a scenario that you are > concering? Let's go back this scenario: fd = open("/some/file", O_RDONLY); ptr1 = mmap(-1, size, PROT_READ, ..., fd, ...); foo1 = *ptr1; There's a read-only PTE at 'ptr1'. Right? The page being pointed to is eligible for LUF via the try_to_unmap() paths. In other words, the page might be reclaimed at any time. If it is reclaimed, the PTE will be cleared. Then, the user might do: munmap(ptr1, PAGE_SIZE); Which will _eventually_ wind up in the zap_pte_range() loop. But that loop will only see pte_none(). It doesn't do _anything_ to the 'struct mmu_gather'. The munmap() then lands in tlb_flush_mmu_tlbonly() where it looks at the 'struct mmu_gather': if (!(tlb->freed_tables || tlb->cleared_ptes || tlb->cleared_pmds || tlb->cleared_puds || tlb->cleared_p4ds)) return; But since there were no cleared PTEs (or anything else) during the unmap, this just returns and doesn't flush the TLB. We now have an address space with a stale TLB entry at 'ptr1' and not even a VMA there. There's nothing to stop a new VMA from going in, installing a *new* PTE, but getting data from the stale TLB entry that still hasn't been flushed.