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Received: from 2001-1c00-8d85-5700-266e-96ff-fe07-7dcc.cable.dynamic.v6.ziggo.nl ([2001:1c00:8d85:5700:266e:96ff:fe07:7dcc] helo=noisy.programming.kicks-ass.net) by casper.infradead.org with esmtpsa (Exim 4.98.2 #2 (Red Hat Linux)) id 1vur1B-0000000GmLl-41uX; Tue, 24 Feb 2026 11:55:58 +0000 Received: by noisy.programming.kicks-ass.net (Postfix, from userid 1000) id 811F930095A; Tue, 24 Feb 2026 12:55:56 +0100 (CET) Date: Tue, 24 Feb 2026 12:55:56 +0100 From: Peter Zijlstra To: Lance Yang Cc: akpm@linux-foundation.org, david@kernel.org, dave.hansen@intel.com, will@kernel.org, aneesh.kumar@kernel.org, npiggin@gmail.com, linux-arch@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH v2 1/1] mm/mmu_gather: replace IPI with synchronize_rcu() when batch allocation fails Message-ID: <20260224115556.GU1395416@noisy.programming.kicks-ass.net> References: <20260224030700.35857-1-lance.yang@linux.dev> <20260224114152.GX1395266@noisy.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260224114152.GX1395266@noisy.programming.kicks-ass.net> X-Stat-Signature: dqnrh9msarb7cwk43ykxkgirj97zoswj X-Rspamd-Server: rspam11 X-Rspam-User: X-Rspamd-Queue-Id: 53A2740006 X-HE-Tag: 1771934162-102902 X-HE-Meta: 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 ioQ/Wae0 dmJ5nMT5LZ24apl1u2qLutcXU28n+ElAYguku1xukuYsSHLqdH0keXtW7BSuP54JUP3+xY/OAoSenAXV1aSfjQEoWx2pKpyJ7tLrdqL92I0lprhBZiCn3fL0aTCTtcqO355nGq8eTBVWWg93v8ohyBj2+Jc9sOBEHGGCyzfibaZaTVkGCU7yJdXjcoX9ZzY0zzIoUIC8+49tV5W9Ba5Mo/j2M7fJZSFWokS4y/aJAByR3vldVEv4NuMUTJTeFNTJmls/MdReamJs6DbBqrnGjHEhVnz8XwKpYu2eS 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 Tue, Feb 24, 2026 at 12:41:52PM +0100, Peter Zijlstra wrote: > On Tue, Feb 24, 2026 at 11:07:00AM +0800, Lance Yang wrote: > > From: Lance Yang > > > > When freeing page tables, we try to batch them. If batch allocation fails > > (GFP_NOWAIT), __tlb_remove_table_one() immediately frees the one without > > batching. > > > > On !CONFIG_PT_RECLAIM, the fallback sends an IPI to all CPUs via > > tlb_remove_table_sync_one(). It disrupts all CPUs even when only a single > > process is unmapping memory. IPI broadcast was reported to hurt RT > > workloads[1]. > > > > tlb_remove_table_sync_one() synchronizes with lockless page-table walkers > > (e.g. GUP-fast) that rely on IRQ disabling. These walkers use > > local_irq_disable(), which is also an RCU read-side critical section. > > > > This patch introduces tlb_remove_table_sync_rcu() which uses RCU grace > > period (synchronize_rcu()) instead of IPI broadcast. This provides the > > same guarantee as IPI but without disrupting all CPUs. Since batch > > allocation already failed, we are in a way slow path where sleeping is > > acceptable - we are in process context (unmap_region, exit_mmap) with only > > mmap_lock held. might_sleep() will catch any invalid context. > > So sending the IPIs also requires non-atomic context, so change there. s/so/no/ -- typing hard