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X-CSE-ConnectionGUID: MnijglncQ2COgh3aXbrcGg== X-CSE-MsgGUID: hz4Hn7OvSQK8Kkgx0QpdRQ== X-IronPort-AV: E=McAfee;i="6600,9927,11082"; a="12898209" X-IronPort-AV: E=Sophos;i="6.08,186,1712646000"; d="scan'208";a="12898209" Received: from fmviesa006.fm.intel.com ([10.60.135.146]) by orvoesa113.jf.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 24 May 2024 10:16:41 -0700 X-CSE-ConnectionGUID: A2uDE7CXTFqPqgJTWs4a4w== X-CSE-MsgGUID: GUTIiLWcSjWUBMGg8FnWvw== X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="6.08,186,1712646000"; d="scan'208";a="34102877" Received: from mgoodin-mobl2.amr.corp.intel.com (HELO [10.125.110.142]) ([10.125.110.142]) by fmviesa006-auth.fm.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 24 May 2024 10:16:40 -0700 Message-ID: <982317c0-7faa-45f0-82a1-29978c3c9f4d@intel.com> Date: Fri, 24 May 2024 10:16:39 -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 Subject: Re: [PATCH v10 00/12] LUF(Lazy Unmap Flush) reducing tlb numbers over 90% To: Byungchul Park , linux-kernel@vger.kernel.org, linux-mm@kvack.org Cc: kernel_team@skhynix.com, akpm@linux-foundation.org, ying.huang@intel.com, 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> From: Dave Hansen Content-Language: en-US Autocrypt: addr=dave.hansen@intel.com; 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charset=UTF-8 Content-Transfer-Encoding: 7bit On 5/9/24 23:51, Byungchul Park wrote: > To achieve that: > > 1. For the folios that map only to non-writable tlb entries, prevent > tlb flush during unmapping but perform it just before the folios > actually become used, out of buddy or pcp. Is this just _pure_ unmapping (like MADV_DONTNEED), or does it apply to changing the memory map, like munmap() itself? > 2. When any non-writable ptes change to writable e.g. through fault > handler, give up luf mechanism and perform tlb flush required > right away. > > 3. When a writable mapping is created e.g. through mmap(), give up > luf mechanism and perform tlb flush required right away. Let's say you do this: fd = open("/some/file", O_RDONLY); ptr1 = mmap(-1, size, PROT_READ, ..., fd, ...); foo1 = *ptr1; You now have a read-only PTE pointing to the first page of /some/file. Let's say try_to_unmap() comes along and decides it can_luf_folio(). The page gets pulled out of the page cache and freed, the PTE is zeroed. But the TLB is never flushed. Now, someone does: fd2 = open("/some/other/file", O_RDONLY); ptr2 = mmap(ptr1, size, PROT_READ, MAP_FIXED, fd, ...); foo2 = *ptr2; and they overwrite the old VMA. Does foo2 have the contents of the new "/some/other/file" or the old "/some/file"? How does the new mmap() know that there was something to flush? BTW, the same thing could happen without a new mmap(). Someone could modify the file in the middle, maybe even from another process. fd = open("/some/file", O_RDONLY); ptr1 = mmap(-1, size, PROT_READ, ..., fd, ...); foo1 = *ptr1; // LUF happens here // "/some/file" changes foo2 = *ptr1; // Does this see the change?