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 BD2A0480DE3 for ; Wed, 22 Jul 2026 11:31:24 +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=1784719889; cv=none; b=Ttk8dqQeXy6GCuyUxx8gWiHdOcBJ9RcNPYmzGLwPLA3LMIuClhqNimbUn03sbAssvTh5nmA/2GYJNt5g6EqzwbmJ8D3xNhbcKzVWsfwK1Nq/lCSx3QYTSLrNsKgdB0rjsU3tXfBF/Pv8+aYaoewiSG89FQ10y5AwGNQrEWcOVlc= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1784719889; c=relaxed/simple; bh=BqG8BwRogCjvef1d7XXFGxwaHrxIngZPuov07GWSN40=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=nfGL3tDPmEbogyTWW4N9wXMfuZp8mrNTdgIwEOLitve/XUH7EEudLaGDbqcqAEyK3wDcdkzMrE7kaepzmUpFl8Yuz9AbfZspHg3SZWsR/Xm5o0XPHPUSjhHPe4fe1W7dudZvtHvtBmol4gbnOzs0sZHBUoYz6BZmM6KE/GkF7U4= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=QFsM7cyW; 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="QFsM7cyW" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 1CB051F000E9; Wed, 22 Jul 2026 11:31:21 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1784719883; bh=6NL9MUSMjOXzXO0PG+O0B+B7O9GtJJNG0e80cL1Tza4=; h=Date:From:To:Cc:Subject:References:In-Reply-To; b=QFsM7cyW63ZHox9yTKsiFzo8dFp8bdo2Fa5APdElwVCoMrDNKWGoBmp1Gx1syXMjA CzHFgDbh6mpJ9ry70GgnCl0GNxZ/QpTNKuvOVogWVAMPh97SCFafJNwXmnkVJJmP7y t6SGz6lgQ1qIbuThkQpnqUpI0kiUGHSnODv17c24AzKvvv0wDPWBIW9XL7OTTl5ttf +jjcRZtag9E3w2IAtcAfHyK+S8BsUY8GMci0GKocwsGmVcNwKUM8/tpzz4Ov89pyvw RyC9zh/MCK/3kpizPUKEljf+2fgwj7y2qUw2tJj9+Cgf5sb3cyedQF7zoPrWzkMzSy J3ZHE5x4Kk1pw== Date: Wed, 22 Jul 2026 12:31:19 +0100 From: Will Deacon To: Yang Shi Cc: catalin.marinas@arm.com, ryan.roberts@arm.com, cl@gentwo.org, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, ljs@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> 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: <255744ec-2a58-492c-a69a-850357440183@os.amperecomputing.com> On Tue, Jun 09, 2026 at 05:06:31PM -0700, Yang Shi wrote: > > > On 6/9/26 2:42 PM, Yang Shi wrote: > > Since commit a166563e7ec3 ("arm64: mm: support large block mapping when > > rodata=full"), the direct mapping may be split on some machines instead > > keeping static since boot. It makes more sense to show the direct mapping > > use in /proc/meminfo than before. > > This patch will make /proc/meminfo show the direct mapping use like the > > below (4K base page size): > > DirectMap4K: 94792 kB > > DirectMap64K: 134208 kB > > DirectMap2M: 1173504 kB > > DirectMap32M: 5636096 kB > > DirectMap1G: 529530880 kB > > > > Although just the machines which support BBML2_NOABORT can split the > > direct mapping, show it on all machines regardless of BBML2_NOABORT so > > that the users have consistent view in order to avoid confusion. > > > > Although ptdump also can tell the direct map use, but it needs to dump > > the whole kernel page table. It is costly and overkilling. It is also > > in debugfs which may not be enabled by all distros. So showing direct > > map use in /proc/meminfo seems more convenient and has less overhead. > > > > Signed-off-by: Yang Shi > > --- > > arch/arm64/mm/mmu.c | 200 +++++++++++++++++++++++++++++++++++++++----- > > 1 file changed, 179 insertions(+), 21 deletions(-) > > > > v8: * Fixed the double accounting per Sashiko > > * Responded the review comments from Sashiko > > v7: * Rebased to v7.1-rc4 > > * Changed "dm" to "lm" to follow ARM convention per Will > > * Used __is_lm_alias() instead of reinventing a new helper per Will > > v6: * Rebased to v7.0-rc3 > > * Rebased on top of Anshuman's v5 "arm64/mm: Enable batched TLB flush > > in unmap_hotplug_range()" > > * Used const for direct map type array per Will > > * Defined PUD size for 16K/64K even though it is not used per Will > > * Removed the misleading comment in init_pmd() per Will > > v5: * Rebased to v6.19-rc4 > > * Fixed the build error for !CONFIG_PROC_FS > > v4: * Used PAGE_END instead of _PAGE_END(VA_BITS_MIN) per Ryan > > * Used shorter name for the helpers and variables per Ryan > > * Fixed accounting for memory hotunplug > > v3: * Fixed the over-accounting problems per Ryan > > * Introduced helpers for add/sub direct map use and #ifdef them with > > CONFIG_PROC_FS per Ryan > > * v3 is a fix patch on top of v2 > > v2: * Counted in size instead of the number of entries per Ryan > > * Removed shift array per Ryan > > * Use lower case "k" per Ryan > > * Fixed a couple of build warnings reported by kernel test robot > > * Fixed a couple of poential miscounts > > Aha, Sashiko is so fast. 2 comments this time. > > #1 > > Will these updates suffer from data races? > > The lm_meminfo array tracks direct mapping statistics and is updated using > > non-atomic += and -= operations. These updates are invoked from multiple > > independent code paths that do not share a common lock. > > For example, runtime page permission changes call > > split_kernel_leaf_mapping_locked() which executes under > > pgtable_split_lock, > > while memory hotplug operations like arch_remove_memory() execute under > > mem_hotplug_lock. Because these paths can run concurrently on different > > CPUs, > > the non-atomic arithmetic could result in data races and lost updates. > > 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/ Will