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X-CSE-ConnectionGUID: FlRXX8cyT3qFjoHK7R9RpA== X-CSE-MsgGUID: YukmpJBxR9CLlyKfMk9h0Q== X-IronPort-AV: E=McAfee;i="6800,10657,11657"; a="68801069" X-IronPort-AV: E=Sophos;i="6.21,190,1763452800"; d="scan'208";a="68801069" Received: from orviesa009.jf.intel.com ([10.64.159.149]) by fmvoesa109.fm.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 30 Dec 2025 20:26:29 -0800 X-CSE-ConnectionGUID: LHHybNlXRt64i4qWKHas1w== X-CSE-MsgGUID: yjyt4zTpSAWybuHlebdsFQ== X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="6.21,190,1763452800"; d="scan'208";a="201107951" Received: from jmaxwel1-mobl.amr.corp.intel.com (HELO [10.125.108.167]) ([10.125.108.167]) by orviesa009-auth.jf.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 30 Dec 2025 20:26:28 -0800 Message-ID: Date: Tue, 30 Dec 2025 20:26:28 -0800 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2 0/3] skip redundant TLB sync IPIs To: Lance Yang , akpm@linux-foundation.org Cc: will@kernel.org, aneesh.kumar@kernel.org, npiggin@gmail.com, peterz@infradead.org, tglx@linutronix.de, mingo@redhat.com, bp@alien8.de, dave.hansen@linux.intel.com, x86@kernel.org, hpa@zytor.com, arnd@arndb.de, david@kernel.org, lorenzo.stoakes@oracle.com, ziy@nvidia.com, baolin.wang@linux.alibaba.com, Liam.Howlett@oracle.com, npache@redhat.com, ryan.roberts@arm.com, dev.jain@arm.com, baohua@kernel.org, ioworker0@gmail.com, shy828301@gmail.com, riel@surriel.com, jannh@google.com, linux-arch@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <20251229145245.85452-1-lance.yang@linux.dev> 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-Rspamd-Server: rspam08 X-Rspamd-Queue-Id: 53393160006 X-Stat-Signature: zgks6cwapq93rzu13tk6548hm14q8czc X-Rspam-User: X-HE-Tag: 1767155191-456820 X-HE-Meta: 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 OBdzx8Uu lSLCEi5d+tyzv/K2dxIc3ecTp56m91EJczoOFvuf5FXnqPsd80ZJa1677U8EenFoZ6i6xhv51D6t9n0pq9V7MGYNYx40zPlKzs/R7Pp7UWNRibgM07v0ZRy9qk3IV6gBLwLURB7JQMH1j203EwJRpC6WTqLWI1qGeGdSRVA66PcifcqY= 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 12/29/25 06:52, Lance Yang wrote: ... > This series introduces a way for architectures to indicate their TLB flush > already provides full synchronization, allowing the redundant IPI to be > skipped. For now, the optimization is implemented for x86 first and applied > to all page table operations that free or unshare tables. I really don't like all the complexity here. Even on x86, there are three or more ways of deriving this. Having the pv_ops check the value of another pv op is also a bit unsettling. That said, complexity can be worth it with sufficient demonstrated gains. But: > When unsharing hugetlb PMD page tables or collapsing pages in khugepaged, > we send two IPIs: one for TLB invalidation, and another to synchronize > with concurrent GUP-fast walkers. Those aren't exactly hot paths. khugepaged is fundamentally rate limited. I don't think unsharing hugetlb PMD page tables just is all that common either. What kind of end user benefit is there to justify the complexity?