From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out-189.mta0.migadu.com (out-189.mta0.migadu.com [91.218.175.189]) (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 9371E37C0ED for ; Tue, 24 Feb 2026 11:32:19 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=91.218.175.189 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1771932741; cv=none; b=UNNsY+HkCjqaGru5sOCpAUB4HqKZSjbjWDFHi0zS/KtvfwXLHAyoViWwzG9U79tXYUDtX5ThpENvGXfSCNn4L5nxPPN/OcIvymQsU/INDYyGs3iNOd2cUp7k3j/ZFrtxAmRlVnHudlx0QBQFH0bOOiOI8EBFTwI7VrlXPmj96Ao= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1771932741; c=relaxed/simple; bh=Qu+46xWBqUHjFm0EuASXZSN6wMzYSfVEYFwhbgRXvUw=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=oCeNN7lzeiq2DAkl0TQ11POdIRDP1UqZakSMvccJ/uYkGKRHA4S4YtY2Fxy1PRhFf5PdN16Dl0gRMZz/p9DNiJE7pKeiqVuKF2IubFbqLPISy5QYLsNCc3DUEf9w9UHWIrmfO6ZnRUQcOvDlF2JPN7KLmBFUd35CEsMH8HFyKys= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev; spf=pass smtp.mailfrom=linux.dev; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b=aGQXpr+L; arc=none smtp.client-ip=91.218.175.189 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=linux.dev Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b="aGQXpr+L" Message-ID: DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1771932737; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=0sUWrbvtvOac8BzVrjFlUEtf6mDEIHrNMXT5s8eGfSo=; b=aGQXpr+LFW5UzYlxmtjZAQfaKo3zEtunIzqMLPndRcd0xxFbuOjoWhJ0xxPlu+itItvZYR P+25P7pa5OxTJwsTEa3ITD0E2r7Nz4kjvl5jtsdytuZSl3rG8uPtk0+Msf9/Yo4qqAmxMt uJ49Hwu6106BiPXR3FnUesDHnVw7NJY= Date: Tue, 24 Feb 2026 19:32:08 +0800 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Subject: Re: [PATCH v2 1/1] mm/mmu_gather: replace IPI with synchronize_rcu() when batch allocation fails Content-Language: en-US To: "David Hildenbrand (Arm)" , 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> X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. From: Lance Yang In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Migadu-Flow: FLOW_OUT On 2026/2/24 19:04, David Hildenbrand (Arm) wrote: > 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 "()" Oops, @Andrew could you drop the "()" here when applying? > > Thanks! > > Acked-by: David Hildenbrand (Arm) Thanks for taking time to review!