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 386B321B908 for ; Fri, 20 Dec 2024 18:30:04 +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=1734719405; cv=none; b=qarK3+zXrCcg/u1uL4t8o/vlRXLRuTga2Trp6Dx2EMEUZ5PCiWRDvT7c2uMRddctvvQ54gI9HwZLzKOPu8qj8J8XBJjdavTo9tJEl5rARL71PAbrEo4mgGS+stWMFjDV/FWeQhP9kKHA2dr7RNxpVjgmydQpTJAXwFxiy/U0qTs= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1734719405; c=relaxed/simple; bh=aIMfz15pdP+WIfj17w163MugP9Q3OvfdVfhj2i8vdVg=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=ggVdLWHaGZNQhZdR/1Lp+T34eF+Cbjxog9mgPDQE2kC7fAWY71NZkn1j38NVNpOcCF4MRf5MaIWBDdwvebn8zI6/3iveegetPyeqxaTCgOmlERgbzuZTOyiAyHuU1dCtsDici1EozxgMxPBKkI4Crf12Vb+FSTZhpCPH0fp/7OI= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 Received: by smtp.kernel.org (Postfix) with ESMTPSA id 620A9C4CECD; Fri, 20 Dec 2024 18:30:02 +0000 (UTC) Date: Fri, 20 Dec 2024 18:30:00 +0000 From: Catalin Marinas To: Zhenhua Huang Cc: will@kernel.org, ardb@kernel.org, ryan.roberts@arm.com, mark.rutland@arm.com, joey.gouly@arm.com, dave.hansen@linux.intel.com, akpm@linux-foundation.org, chenfeiyang@loongson.cn, chenhuacai@kernel.org, linux-mm@kvack.org, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, Anshuman Khandual Subject: Re: [PATCH v2 1/2] arm64: mm: vmemmap populate to page level if not section aligned Message-ID: References: <20241209094227.1529977-1-quic_zhenhuah@quicinc.com> <20241209094227.1529977-2-quic_zhenhuah@quicinc.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: <20241209094227.1529977-2-quic_zhenhuah@quicinc.com> On Mon, Dec 09, 2024 at 05:42:26PM +0800, Zhenhua Huang wrote: > Commit c1cc1552616d ("arm64: MMU initialisation") > optimizes the vmemmap to populate at the PMD section level. However, if > start or end is not aligned to a section boundary, such as when a > subsection is hot added, populating the entire section is wasteful. For > instance, if only one subsection hot-added, the entire section's struct > page metadata will still be populated.In such cases, it is more effective > to populate at page granularity. OK, so from the vmemmap perspective, we waste up to 2MB memory that has been allocated even if a 2MB hot-plugged subsection required only 32KB of struct page. I don't mind this much really. I hope all those subsections are not scattered around to amplify this waste. > This change also addresses mismatch issues during vmemmap_free(): When > pmd_sect() is true, the entire PMD section is cleared, even if there is > other effective subsection. For example, pagemap1 and pagemap2 are part > of a single PMD entry and they are hot-added sequentially. Then pagemap1 > is removed, vmemmap_free() will clear the entire PMD entry, freeing the > struct page metadata for the whole section, even though pagemap2 is still > active. I think that's the bigger issue. We can't unplug a subsection only. Looking at unmap_hotplug_pmd_range(), it frees a 2MB vmemmap section but that may hold struct page for the equivalent of 128MB of memory. So any struct page accesses for the other subsections will fault. > Fixes: c1cc1552616d ("arm64: MMU initialisation") I wouldn't add a fix for the first commit adding arm64 support, we did not even have memory hotplug at the time (added later in 5.7 by commit bbd6ec605c0f ("arm64/mm: Enable memory hot remove")). IIUC, this hasn't been a problem until commit ba72b4c8cf60 ("mm/sparsemem: support sub-section hotplug"). That commit broke some arm64 assumptions. > Signed-off-by: Zhenhua Huang > --- > arch/arm64/mm/mmu.c | 4 +++- > 1 file changed, 3 insertions(+), 1 deletion(-) > > diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c > index e2739b69e11b..fd59ee44960e 100644 > --- a/arch/arm64/mm/mmu.c > +++ b/arch/arm64/mm/mmu.c > @@ -1177,7 +1177,9 @@ int __meminit vmemmap_populate(unsigned long start, unsigned long end, int node, > { > WARN_ON((start < VMEMMAP_START) || (end > VMEMMAP_END)); > > - if (!IS_ENABLED(CONFIG_ARM64_4K_PAGES)) > + if (!IS_ENABLED(CONFIG_ARM64_4K_PAGES) || > + !IS_ALIGNED(page_to_pfn((struct page *)start), PAGES_PER_SECTION) || > + !IS_ALIGNED(page_to_pfn((struct page *)end), PAGES_PER_SECTION)) > return vmemmap_populate_basepages(start, end, node, altmap); > else > return vmemmap_populate_hugepages(start, end, node, altmap); An alternative would be to fix unmap_hotplug_pmd_range() etc. to avoid nuking the whole vmemmap pmd section if it's not empty. Not sure how easy that is, whether we have the necessary information (I haven't looked in detail). A potential issue - can we hotplug 128MB of RAM and only unplug 2MB? If that's possible, the problem isn't solved by this patch. -- Catalin