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 644A823A9BA; Thu, 13 Feb 2025 17:56:59 +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=1739469419; cv=none; b=ORWWxP+Uh330omxq5KitABzOF/kQESzz/lrfwS9BuCvxFxykQ6/sdtkoZfkxGJDCmsOt4fr10V6mmi/bh6PfAXTqIvlB/FjunSqyHLE+ND5akkspmiahMcAgYfHXx7OTAZWL2QpbaUTFyrNBHH0LGzUNJ2/LdsYFs8cMnYISC8Q= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1739469419; c=relaxed/simple; bh=0WV2CmcTZUiH+zQk/eImcSLFCn6EHvq25FoHekBJf6c=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=Ha6eaXpPiA+MVWaYPHynquYne2hrTGsL2/5eb8ZxObU5uCfas3uwsuL4/JYn1oj1SLkUShR8dK1iVeHmtvI7xnCzyBfK8arDtTf+mpSZ+g7ELjXus6MsahgubF1pEwF3LeysjKrjaPOiSZSWjZVceFIIe7eyVnn3IhppqVbdWtU= 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 220B4C4CED1; Thu, 13 Feb 2025 17:56:55 +0000 (UTC) Date: Thu, 13 Feb 2025 17:56:53 +0000 From: Catalin Marinas To: David Hildenbrand Cc: Zhenhua Huang , anshuman.khandual@arm.com, 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, quic_tingweiz@quicinc.com, stable@vger.kernel.org Subject: Re: [PATCH v6] arm64: mm: Populate vmemmap/linear at the page level for hotplugged sections Message-ID: References: <20250213075703.1270713-1-quic_zhenhuah@quicinc.com> <9bc91fe3-c590-48e2-b29f-736d0b056c34@redhat.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 Thu, Feb 13, 2025 at 05:16:37PM +0100, David Hildenbrand wrote: > On 13.02.25 16:49, Catalin Marinas wrote: > > On Thu, Feb 13, 2025 at 01:59:25PM +0100, David Hildenbrand wrote: > > > On 13.02.25 08:57, Zhenhua Huang wrote: > > > > On the arm64 platform with 4K base page config, SECTION_SIZE_BITS is set > > > > to 27, making one section 128M. The related page struct which vmemmap > > > > points to is 2M then. > > > > Commit c1cc1552616d ("arm64: MMU initialisation") optimizes the > > > > vmemmap to populate at the PMD section level which was suitable > > > > initially since hot plug granule is always one section(128M). However, > > > > commit ba72b4c8cf60 ("mm/sparsemem: support sub-section hotplug") > > > > introduced a 2M(SUBSECTION_SIZE) hot plug granule, which disrupted the > > > > existing arm64 assumptions. > > > > > > > > Considering the vmemmap_free -> unmap_hotplug_pmd_range path, when > > > > pmd_sect() is true, the entire PMD section is cleared, even if there is > > > > other effective subsection. For example page_struct_map1 and > > > > page_strcut_map2 are part of a single PMD entry and they are hot-added > > > > sequentially. Then page_struct_map1 is removed, vmemmap_free() will clear > > > > the entire PMD entry freeing the struct page map for the whole section, > > > > even though page_struct_map2 is still active. Similar problem exists > > > > with linear mapping as well, for 16K base page(PMD size = 32M) or 64K > > > > base page(PMD = 512M), their block mappings exceed SUBSECTION_SIZE. > > > > Tearing down the entire PMD mapping too will leave other subsections > > > > unmapped in the linear mapping. > > > > > > > > To address the issue, we need to prevent PMD/PUD/CONT mappings for both > > > > linear and vmemmap for non-boot sections if corresponding size on the > > > > given base page exceeds SUBSECTION_SIZE(2MB now). > > > > > > > > Cc: # v5.4+ > > > > Fixes: ba72b4c8cf60 ("mm/sparsemem: support sub-section hotplug") > > > > Reviewed-by: Catalin Marinas > > > > Signed-off-by: Zhenhua Huang > > > > > > Just so I understand correctly: for ordinary memory-sections-size hotplug > > > (NVDIMM, virtio-mem), we still get a large mapping where possible? > > > > Up to 2MB blocks only since that's the SUBSECTION_SIZE value. The > > vmemmap mapping is also limited to PAGE_SIZE mappings (we could use > > contiguous mappings for vmemmap but it's not wired up; I don't think > > it's worth the hassle). > > But that's messed up, no? > > If someone hotplugs a memory section, they have to hotunplug a memory > section, not parts of it. > > That's why x86 does in vmemmap_populate(): > > if (end - start < PAGES_PER_SECTION * sizeof(struct page)) > err = vmemmap_populate_basepages(start, end, node, NULL); > else if (boot_cpu_has(X86_FEATURE_PSE)) > err = vmemmap_populate_hugepages(start, end, node, altmap); > ... > > Maybe I'm missing something. Most importantly, why the weird subsection > stuff is supposed to degrade ordinary hotplug of dimms/virtio-mem etc. I think that's based on the discussion for a previous version assuming that the hotplug/unplug sizes are not guaranteed to be symmetric: https://lore.kernel.org/lkml/a720aaa5-a75e-481e-b396-a5f2b50ed362@quicinc.com/ If that's not the case, we can indeed ignore the SUBSECTION_SIZE altogether and just rely on the start/end of the hotplugged region. -- Catalin