From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (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 74D62384CD1 for ; Tue, 28 Jul 2026 11:37:20 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1785238641; cv=none; b=ePkPStKedqtTnA4BSb8fAjBZh2LNgWv++F1BNqvmxUMmjFluugeguvacF/V5rpKTSjeA8ME8SXqcgESA/dNmeRkz6mQjAe6QOBkQYXnfi2V+fNP9JeYdHH+Mr/ZUGcu3A8eg81zzv3H/D+2GboL76PMsiGSnU1LQydXa1tgJ1+4= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1785238641; c=relaxed/simple; bh=WlmiKyS45RjP6Jk0Unm1wk0Df9MNbq2BvK85BROaIyw=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=aK5XP3VyVvvltnkzgV1MwsdDOpYv9pLj4EHClJCsXVtqlp3owmh3jXi4K2bFeoxZsOkcadXtmoPKDh5ttWKo9rWPeFDRYDrZfAHwADpPTbmAUiy9RwUtq5fYRcLPKJJkdpipleixF7FDfPLlMW8rET7CyCpXAf18wn6pkBsbaco= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=FFLzm+Bz; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="FFLzm+Bz" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 3C1341F000E9; Tue, 28 Jul 2026 11:37:18 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1785238640; bh=WlmiKyS45RjP6Jk0Unm1wk0Df9MNbq2BvK85BROaIyw=; h=Date:From:To:Cc:Subject:References:In-Reply-To; b=FFLzm+BzjamxA0umMXZlmuy+NQsVtww3sldK4LN7vydtjSnFD4E4o5dIIOBj8e1bY YoWvUEqv5NQCMaScNjoZNdpiwgZbVVgx4pXSdq/Qae0T3nP26jYLOsVLc3CRBOGq2Q UEbjPr0XBd7Mo0PFQlpXYuoPSR/MBtdsUSFthkR2BUVpeg3MxOkFhhrfLqiIk7YyGR DZjqZe5f0NnUHyIGlL+RSFcek4krfJTe0f+qSrerxp+amAabhndHKYFvGheBgcfX/5 ZksOWw4iudVYkxxkpxe67+7BOwE0wOlTW4aV6Cbti2QFth58S22+sFOP/yt5ndjKob Dg9rjkEudxLpQ== Date: Tue, 28 Jul 2026 12:37:03 +0100 From: "Lorenzo Stoakes (ARM)" To: Will Deacon Cc: Yang Shi , catalin.marinas@arm.com, ryan.roberts@arm.com, cl@gentwo.org, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org Subject: Re: [v8 PATCH] arm64: mm: show direct mapping use in /proc/meminfo Message-ID: References: <20260609214205.1260279-1-yang@os.amperecomputing.com> <255744ec-2a58-492c-a69a-850357440183@os.amperecomputing.com> <79d7069b-2895-4be3-a2a0-327ad3444bf3@os.amperecomputing.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: On Tue, Jul 28, 2026 at 10:27:17AM +0100, Will Deacon wrote: > On Mon, Jul 27, 2026 at 12:50:59PM -0700, Yang Shi wrote: > > On 7/22/26 4:31 AM, Will Deacon wrote: > > > > Yes, it may race with memory hotplug. I missed memory hotplug for v7 > > > > Sashiko. Two options to solve it: > > > > > > > > 1. Use atomic variables. Make lm_meminfo[NR_LM_TYPE] atomic_long_t, then > > > > manipulate it with atomic ops. > > > > 2. Protect it with a spin lock. > > > > > > > > The contention for the cache line or the spin lock should be rare since > > > > memory hotplug should happen rarely. Any preference? > > > I'd vote for keeping it simple and using a lock. Might be worth looking > > > at the ongoing work from Lorenzo: > > > > > > https://lore.kernel.org/all/20260717-series-vmap-race-fix-v5-0-606a0ac6d3e5@kernel.org/ > > > > Thanks, Will. Took a look at Lorenzo's work. IIUC he used init_mm mmap_lock > > to protect vmalloc area and linear mapping collapse on x86 in order to > > serialize against ptdump. I don't think we should use init_mm mmap_lock. We > > want to serialize linear mapping counter update between split and memory > > hotplug, but neither of them takes init_mm mmap_lock. We can let them take > > init_mm mmap_lock, but it sounds too overkilling. > > Hmm, but if split and hotplug don't take the mmap_lock, how are they > serialised against ptdump? From what I understood, Lorenzo's patches > change ptdump to take it for write in order to serialise against other > walkers. As for memory hotplug - ptdump already takes that lock in ptdump_walk_pgd() so you'd need to add that on split I think? So I think you need to take it in the split code and in arch_report_meminfo. So that solves that, you can get ptdump serialisation by ALSO taking the init_mm read map on split. That way you can rely on core ptdump stuff without having to add new locking in arm64-specific ptdump. And would you also need the pgtable_split_lock in arch_report_meminfo() to serialise lm_meminfo access? (Side-note: I am going to try to implement RCU kernel page table freeing at some point... so then maybe ptdump won't need the init_mm lock any more. But we could still take it there for this at least :) > > Will Cheers, Lorenzo