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X-CSE-ConnectionGUID: vSQ4Q/I5TNqpBKJ0JKQCPQ== X-CSE-MsgGUID: RNlxKBuWTLqmPWm1pyln3g== X-IronPort-AV: E=McAfee;i="6800,10657,11766"; a="80611089" X-IronPort-AV: E=Sophos;i="6.23,196,1770624000"; d="scan'208";a="80611089" Received: from fmviesa005.fm.intel.com ([10.60.135.145]) by fmvoesa107.fm.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 24 Apr 2026 08:52:25 -0700 X-CSE-ConnectionGUID: E0Nx+DioSqmBoBKTm4PzUg== X-CSE-MsgGUID: czNrzks6Rq6tARJvsoUAxw== X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="6.23,196,1770624000"; d="scan'208";a="237987969" Received: from sghuge-mobl2.amr.corp.intel.com (HELO [10.125.108.73]) ([10.125.108.73]) by fmviesa005-auth.fm.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 24 Apr 2026 08:52:22 -0700 Message-ID: Date: Fri, 24 Apr 2026 08:52:16 -0700 Precedence: bulk X-Mailing-List: linux-arch@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 7.2 v10 1/2] mm/mmu_gather: prepare to skip redundant sync IPIs To: Peter Zijlstra , Lance Yang Cc: akpm@linux-foundation.org, david@kernel.org, dave.hansen@linux.intel.com, ypodemsk@redhat.com, hughd@google.com, will@kernel.org, aneesh.kumar@kernel.org, npiggin@gmail.com, tglx@linutronix.de, mingo@redhat.com, bp@alien8.de, x86@kernel.org, hpa@zytor.com, arnd@arndb.de, ljs@kernel.org, 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, shy828301@gmail.com, riel@surriel.com, jannh@google.com, jgross@suse.com, seanjc@google.com, pbonzini@redhat.com, boris.ostrovsky@oracle.com, virtualization@lists.linux.dev, kvm@vger.kernel.org, linux-arch@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, ioworker0@gmail.com References: <20260424062528.71951-1-lance.yang@linux.dev> <20260424062528.71951-2-lance.yang@linux.dev> <20260424150454.GF3126523@noisy.programming.kicks-ass.net> 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 4/24/26 08:04, Peter Zijlstra wrote: > So I don't like this at all.... The comment says there is a preceding > TLB flush, but there is nothing that guarantees there is. One would have > to go audit all users and ensure this is always true. > > This thing is incredibly fragile. Yeah, this seems like an attempt to apply a code solution to a data structure problem. I think I talked about this in earlier iterations. But, ideally, what happens here is that the things doing the table freeing or collapsing or whatever would note in a data structure what they did. Then the actual flushing code can look at the data structure and figure out what kind of flush it needs. Things like "do I need to flush on lazy CPUs?" Or, "have I done an IPI since the last page table free?" But, if I remember from earlier in this thread, some of the callers of this stuff didn't have a nice data structure (like an mmu_gather) passed in to the places where it would be needed to exfiltrate the information. I think Lance gave up on that because it looked too invasive to him. But, I think this boils down to the code being too fragile as-is to support what Lance is trying to do. It actually needs some refactoring love before it can support the desired optimization. I'm not sure there's an easy way out here.