From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 9B493366DD7; Tue, 24 Feb 2026 11:04:08 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1771931048; cv=none; b=E79m7XfpxyQ/Q1ppbeAlxelcJ7xuH8Jph9TtuXw5PGACmfl2PX7KRKkqDps58YItj+BN1WEHYttcztHPJy+G6QRq4G0Q389M0B/oJpf61fV8Rb9vjvCZskwRUXTxwkJumHFaJdKe8dj++sp4AJcHcterV7He0/MEaNvtONF/8/8= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1771931048; c=relaxed/simple; bh=93TFsFhUI4si2PRVVTQOu/OozcSIrJzAXBGMvSoUCVM=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=TFmbin4oEiZYuDvl5bc53pw6xYIQzBRjpH8YhRPxPuAdqTJz+FOEHhvZz6MoA7XeoIzxEXPadQfI58dcv+I3XxzFQldJQ3HVyWY8SrISygHt6kLd0t33Zsv7hkyplP4WxAtveBXummj/1OW45kpv2cvVtjj6XfRx6p46c2iv2C4= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=mxXNFSZE; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="mxXNFSZE" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 439E0C19424; Tue, 24 Feb 2026 11:04:06 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1771931048; bh=93TFsFhUI4si2PRVVTQOu/OozcSIrJzAXBGMvSoUCVM=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=mxXNFSZEgof4oWvDh69w8cl9UASs5pzSlvh6ePsgKJIA+Z4aZCoL8o/NzkhyaACAq jVb4bMPUBDBV4zV6B26G80K1qFX4PaqMs1eJmaEuv8v4fp3OJNczMtMN5h7OHpf9K6 Ial2n412Zt46Wl2/lTWfypPx6ZgyKD5BpLTPyghJ4foYVp6adWPm5ojdyV528uVulg wpoDa8jWI3nScevOBgmucmnWsFCqqLq7z8Dt4SrhuQgL37L45+sgWCvaUXLQ7K/j1s vS5ZTkyFevwbM74GQeKMZPo5VcFiwzGgbDhSCm9NaUzvI7jtZLuCc+m0xgYygbCVkh XQV9aGj7Kb4pQ== Message-ID: Date: Tue, 24 Feb 2026 12:04:00 +0100 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 v2 1/1] mm/mmu_gather: replace IPI with synchronize_rcu() when batch allocation fails To: Lance Yang , akpm@linux-foundation.org, peterz@infradead.org Cc: 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 References: <20260224030700.35857-1-lance.yang@linux.dev> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit On 2/24/26 04:07, 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. > > [1] https://lore.kernel.org/linux-mm/1b27a3fa-359a-43d0-bdeb-c31341749367@kernel.org/ > > Link: https://lore.kernel.org/linux-mm/20260202150957.GD1282955@noisy.programming.kicks-ass.net/ > Link: https://lore.kernel.org/linux-mm/dfdfeac9-5cd5-46fc-a5c1-9ccf9bd3502a@intel.com/ > Link: https://lore.kernel.org/linux-mm/bc489455-bb18-44dc-8518-ae75abda6bec@kernel.org/ > Suggested-by: Peter Zijlstra > Suggested-by: Dave Hansen > Suggested-by: David Hildenbrand (Arm) > Signed-off-by: Lance Yang > --- > v1 -> v2: > - Wrap synchronize_rcu() in tlb_remove_table_sync_rcu() with proper > kerneldoc (per David) > - Add might_sleep() to make sleeping constraint explicit (per Dave) > - Clarify this is for synchronization, not memory freeing (per Dave) > - https://lore.kernel.org/linux-mm/20260223033604.10198-1-lance.yang@linux.dev/ > > include/asm-generic/tlb.h | 4 ++++ > mm/mmu_gather.c | 22 +++++++++++++++++++++- > 2 files changed, 25 insertions(+), 1 deletion(-) > > diff --git a/include/asm-generic/tlb.h b/include/asm-generic/tlb.h > index 4aeac0c3d3f0..bdcc2778ac64 100644 > --- a/include/asm-generic/tlb.h > +++ b/include/asm-generic/tlb.h > @@ -251,6 +251,8 @@ static inline void tlb_remove_table(struct mmu_gather *tlb, void *table) > > void tlb_remove_table_sync_one(void); > > +void tlb_remove_table_sync_rcu(void); > + > #else > > #ifdef tlb_needs_table_invalidate > @@ -259,6 +261,8 @@ void tlb_remove_table_sync_one(void); > > static inline void tlb_remove_table_sync_one(void) { } > > +static inline void tlb_remove_table_sync_rcu(void) { } > + > #endif /* CONFIG_MMU_GATHER_RCU_TABLE_FREE */ > > > diff --git a/mm/mmu_gather.c b/mm/mmu_gather.c > index fe5b6a031717..2c6fa8db55df 100644 > --- a/mm/mmu_gather.c > +++ b/mm/mmu_gather.c > @@ -296,6 +296,26 @@ static void tlb_remove_table_free(struct mmu_table_batch *batch) > call_rcu(&batch->rcu, tlb_remove_table_rcu); > } > > +/** > + * tlb_remove_table_sync_rcu() - synchronize with software page-table walkers Nit: no need for the "()" Thanks! Acked-by: David Hildenbrand (Arm) -- Cheers, David