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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 desiato.infradead.org with esmtpsa (Exim 4.98.2 #2 (Red Hat Linux)) id 1vurdg-0000000761b-0wfK; Tue, 24 Feb 2026 12:35:44 +0000 Received: by noisy.programming.kicks-ass.net (Postfix, from userid 1000) id 8E54830095A; Tue, 24 Feb 2026 13:35:38 +0100 (CET) Date: Tue, 24 Feb 2026 13:35:38 +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: <20260224123538.GW1282955@noisy.programming.kicks-ass.net> References: <20260224030700.35857-1-lance.yang@linux.dev> <20260224114152.GX1395266@noisy.programming.kicks-ass.net> <5cc90e7a-ae32-4352-8e0b-2eee5a5ee122@linux.dev> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <5cc90e7a-ae32-4352-8e0b-2eee5a5ee122@linux.dev> X-Rspamd-Server: rspam09 X-Stat-Signature: kg8z4nzt38fbo9bzcenfnbqh6xd7s1jh X-Rspamd-Queue-Id: 86F2E80011 X-Rspam-User: X-HE-Tag: 1771936546-881473 X-HE-Meta: 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 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 08:18:46PM +0800, Lance Yang wrote: > > > On 2026/2/24 19:41, 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. > > Yeah, you're right! > > > What isn't explained, and very much not clear to me, is why > > tlb_remove_table_sync_one() is retained? > > Good point. tlb_remove_table_sync_one() is still needed in: > > 1) khugepaged (mm/khugepaged.c) - after pmdp_collapse_flush() > 2) tlb_finish_mmu() (tlb.h) - when tlb->fully_unshared_tables > 3) ... > > These are not slow paths like batch allocation failure. This patch only > converts this obvious slow path first. > > I'm working on converting the remaining callers as well, but not with > RCU, looking at other options (e.g. targeted IPI). OK, so with that addition to the Changelog, Acked-by: Peter Zijlstra (Intel)