From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out-173.mta0.migadu.com (out-173.mta0.migadu.com [91.218.175.173]) (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 C04F641C63 for ; Mon, 10 Nov 2025 04:30:30 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=91.218.175.173 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1762749032; cv=none; b=k0KE872OXo7qVWKDPAZaJPe0hc/kvqw88F1g5aQdAW5nKhkyfUB+oJQM0NIHWXPxu5yG8V90I3yaDSkHZ9KR9OIxNIkB2KXOOx43crT5qBhHoJxhZKJgsKvoEAGzlpI0JyA9engjC1Q+PcWtCZ+7gZZN2dGrIbEEgXkZ77AZ/tY= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1762749032; c=relaxed/simple; bh=j3o1PVMLua/Gnsl0SBuyqCBAYrO7OxpEAe/BvmQDT1I=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=jMw/3ig+x9U7cAW5u9kJf4C45EVtwB7WWEGS6V4d7SgAWATTcY+MpWZSXhw5G5veBPZGPf6h25EnCiA1b1Jmsy2A5/XogwoZIjzFcULsrJbe+L6SWZkzh/6iMKJ1ps/o8D8CtHXADXrfMVYuryswHzveTUEsafHqDIQgzlV7uS8= 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=QrKMgcjb; arc=none smtp.client-ip=91.218.175.173 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="QrKMgcjb" Message-ID: <2a68bddf-e6e6-4960-b5bc-1a39d747ea9b@linux.dev> DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1762749017; 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=VM4mtZug8xE4ghkhPgkEdlWdDR6+a0E4+YRze3eMGSI=; b=QrKMgcjb+M+1F+KOE1U1Kz2oPhiC+29MYWaxjYCseEb9dCjF/qm7ht2OVhV3hu6Ni8yXAS 0k2iLbwHoKMIxa4K4ZUwxl+IOfLx4O354NnSpcjicDEqtBtrjS0hjI2H/wZRxnioVCq0s9 yXbOo+3DumqD4576zhhXeGXKJJNoBrE= Date: Mon, 10 Nov 2025 12:30:06 +0800 Precedence: bulk X-Mailing-List: cgroups@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Subject: Re: [PATCH v1 04/26] mm: vmscan: refactor move_folios_to_lru() To: Harry Yoo , Shakeel Butt Cc: hannes@cmpxchg.org, hughd@google.com, mhocko@suse.com, roman.gushchin@linux.dev, muchun.song@linux.dev, david@redhat.com, lorenzo.stoakes@oracle.com, ziy@nvidia.com, imran.f.khan@oracle.com, kamalesh.babulal@oracle.com, axelrasmussen@google.com, yuanchu@google.com, weixugc@google.com, akpm@linux-foundation.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, cgroups@vger.kernel.org, Muchun Song , Qi Zheng , Sebastian Andrzej Siewior , Clark Williams , Steven Rostedt , linux-rt-devel@lists.linux.dev References: <97ea4728568459f501ddcab6c378c29064630bb9.1761658310.git.zhengqi.arch@bytedance.com> <366385a3-ed0e-440b-a08b-9cf14165ee8f@linux.dev> X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. From: Qi Zheng In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Migadu-Flow: FLOW_OUT On 11/10/25 10:13 AM, Harry Yoo wrote: > On Fri, Nov 07, 2025 at 10:32:52PM -0800, Shakeel Butt wrote: >> On Fri, Nov 07, 2025 at 10:20:57PM +0900, Harry Yoo wrote: >>> >>> Although it's mentioned in the locking documentation, I'm afraid that >>> local_lock is not the right interface to use here. Preemption will be >>> disabled anyway (on both PREEMPT_RT and !PREEMPT_RT) when the stats are >>> updated (in __mod_node_page_state()). >>> >>> Here we just want to disable IRQ only on !PREEMPT_RT (to update >>> the stats safely). >> >> I don't think there is a need to disable IRQs. There are three stats >> update functions called in that hunk. >> >> 1) __mod_lruvec_state >> 2) __count_vm_events >> 3) count_memcg_events >> >> count_memcg_events() can be called with IRQs. __count_vm_events can be >> replaced with count_vm_events. > > Right. > >> For __mod_lruvec_state, the >> __mod_node_page_state() inside needs preemption disabled. > > The function __mod_node_page_state() disables preemption. > And there's a comment in __mod_zone_page_state(): > >> /* >> * Accurate vmstat updates require a RMW. On !PREEMPT_RT kernels, >> * atomicity is provided by IRQs being disabled -- either explicitly >> * or via local_lock_irq. On PREEMPT_RT, local_lock_irq only disables >> * CPU migrations and preemption potentially corrupts a counter so >> * disable preemption. >> */ >> preempt_disable_nested(); > > We're relying on IRQs being disabled on !PREEMPT_RT. So it's possible for us to update vmstat within an interrupt context, right? There is also a comment above __mod_zone_page_state(): /* * For use when we know that interrupts are disabled, * or when we know that preemption is disabled and that * particular counter cannot be updated from interrupt context. */ BTW, the comment inside __mod_node_page_state() should be: /* See __mod_zone_page_state */ instead of /* See __mod_node_page_state */ Will fix it. > > Maybe we could make it safe against re-entrant IRQ handlers by using > read-modify-write operations? Isn't it because of the RMW operation that we need to use IRQ to guarantee atomicity? Or have I misunderstood something? >