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 0B13F41B361 for ; Mon, 2 Mar 2026 15:51:35 +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=1772466696; cv=none; b=nyEV8gt7o3yxYCSCyOsAWpxkzCJzLdmSBuVx+0W5wGatLlum2lN+xN+JiUUoTluf7bOK8lxKES9vBOC/HCnN/VAg6Ey8s1BcuW7ai19w+H6STEkawNLbmwhllFssJdZIRZ3a6d3pktSflVoeu43UkEPvqhAthZafOOqGGCxavLA= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1772466696; c=relaxed/simple; bh=5yZdFyABlRn8cWdlXJGBlydbmpCFPmAJ9Eli7s9/mAU=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=PA/j5lUjGFeAs7YIw161sU5LPlJdh1Jne1r9YdXEi7fpQYM9VhZzWV0sKv43xOu82UWfXYNe2/SKVvHxMnB4oxavFO9+C5/zs4SPpbyy5tkY66mxQtPcq8yjxfeYMPCk+PPr3e4Y1S5TF5LEcJcRSlAcNmvLGMbSSfWdtmz1slA= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=fjkCeGfP; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="fjkCeGfP" Received: by smtp.kernel.org (Postfix) with ESMTPSA id D497FC2BC87; Mon, 2 Mar 2026 15:51:32 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1772466695; bh=5yZdFyABlRn8cWdlXJGBlydbmpCFPmAJ9Eli7s9/mAU=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=fjkCeGfPyXSNyM/LdAgApX4BOf2fVdPtk52hASkj1ULq+2/48rb9H3frtcnjxwVHh EOnLbzgfEByiENr2UASkhvadsPs0bhv8jW69TM167ZMOhGWRERsAOnoHmIO8vJq6KD ZLelILP0BIN1ejtXgu/ehsisE7FeM6djX4nREtpA8QdyTSDttXhjdisWQqIjU/DljJ pJar4dWGEH4rW7wQe/J1IPoSzYAuqqiAmHeUxEIVRlywY4K+E4rK2CuzkXphnaP+/j 9dBfELdNUJomkOGtX4/sr0/6wmoDXh80UVqOZTaMeQ2ivm5WX9UaYcYkSy+U4LfsDU /YBuVBpgmyzPQ== Message-ID: <6eee7f14-c3f2-4e16-b713-65c2ea141457@kernel.org> Date: Mon, 2 Mar 2026 16:51:30 +0100 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH V3 2/2] arm64/mm: Reject memory removal that splits a kernel leaf mapping To: Anshuman Khandual , linux-arm-kernel@lists.infradead.org Cc: mark.rutland@arm.com, Catalin Marinas , Will Deacon , Ryan Roberts , Yang Shi , Christoph Lameter , linux-kernel@vger.kernel.org References: <20260224062423.972404-1-anshuman.khandual@arm.com> <20260224062423.972404-3-anshuman.khandual@arm.com> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit On 2/24/26 07:24, Anshuman Khandual wrote: > Linear and vmemmap mappings that get teared down during a memory hot remove s/teared/torn/ ? > operation might contain leaf level entries on any page table level. If the > requested memory range's linear or vmemmap mappings falls within such leaf > entries, new mappings need to be created for the remaining memory mapped on > the leaf entry earlier, following standard break before make aka BBM rules. > But kernel cannot tolerate BBM and hence remapping to fine grained leaves > would not be possible on systems without BBML2_NOABORT. > > Currently memory hot remove operation does not perform such restructuring, > and so removing memory ranges that could split a kernel leaf level mapping > need to be rejected. > > While memory_hotplug.c does appear to permit hot removing arbitrary ranges > of memory, the higher layers that drive memory_hotplug (e.g. ACPI, virtio, > ...) all appear to treat memory as fixed size devices. So it is impossible > to hot unplug a different amount than was previously hot plugged, and hence > we should never see a rejection in practice, but adding the check makes us > robust against a future change. Is this then really a fix that warrens? Closes: https://lore.kernel.org/all/aWZYXhrT6D2M-7-N@willie-the-truck/ Fixes: bbd6ec605c0f ("arm64/mm: Enable memory hot remove") ... if nothing is broken? I can understand the desire for this check, but I am confused about Fixes: > > Cc: Catalin Marinas > Cc: Will Deacon > Cc: linux-arm-kernel@lists.infradead.org > Cc: linux-kernel@vger.kernel.org > Closes: https://lore.kernel.org/all/aWZYXhrT6D2M-7-N@willie-the-truck/ > Fixes: bbd6ec605c0f ("arm64/mm: Enable memory hot remove") > Reviewed-by: Ryan Roberts > Suggested-by: Ryan Roberts > Signed-off-by: Anshuman Khandual > --- > arch/arm64/mm/mmu.c | 155 ++++++++++++++++++++++++++++++++++++++++++-- > 1 file changed, 149 insertions(+), 6 deletions(-) > > diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c > index dfb61d218579..ce0b966f42d1 100644 > --- a/arch/arm64/mm/mmu.c > +++ b/arch/arm64/mm/mmu.c > @@ -2063,6 +2063,142 @@ void arch_remove_memory(u64 start, u64 size, struct vmem_altmap *altmap) > __remove_pgd_mapping(swapper_pg_dir, __phys_to_virt(start), size); > } > > + > +static bool addr_splits_kernel_leaf(unsigned long addr) > +{ > + pgd_t *pgdp, pgd; > + p4d_t *p4dp, p4d; > + pud_t *pudp, pud; > + pmd_t *pmdp, pmd; > + pte_t *ptep, pte; > + > + /* > + * PGD level: I consider all these "XXX level:" comments unhelpful. It's mostly there, in the code already: PGDIR_SIZE, PGDIR_SIZE, PUD_SIZE ... :) > + * > + * If addr is PGD_SIZE aligned - already on a leaf boundary Similarly, repeating that is not particularly helpful. You could have a single comment at the very top that says "If the given address points at a the start address of a possible leaf, we certainly won't split. Otherwise, check if we would actually split a leaf by traversing the page tables further." > + */ > + if (ALIGN_DOWN(addr, PGDIR_SIZE) == addr) if (IS_ALIGNED(addr, PGDIR_SIZE)) return false; ? > + return false; > + > + pgdp = pgd_offset_k(addr); > + pgd = pgdp_get(pgdp); > + if (!pgd_present(pgd)) > + return false; How could we end up with non-present areas in a range we hotplugged earlier? > + > + /* > + * P4D level: > + * > + * If addr is P4D_SIZE aligned - already on a leaf boundary > + */ > + if (ALIGN_DOWN(addr, P4D_SIZE) == addr) > + return false; > + > + p4dp = p4d_offset(pgdp, addr); > + p4d = p4dp_get(p4dp); > + if (!p4d_present(p4d)) > + return false; > + > + /* > + * PUD level: > + * > + * If addr is PUD_SIZE aligned - already on a leaf boundary > + */ > + if (ALIGN_DOWN(addr, PUD_SIZE) == addr) > + return false; > + > + pudp = pud_offset(p4dp, addr); > + pud = pudp_get(pudp); > + if (!pud_present(pud)) > + return false; > + > + if (pud_leaf(pud)) > + return true; > + > + /* > + * CONT_PMD level: > + * > + * If addr is CONT_PMD_SIZE aligned - already on a leaf boundary > + */ > + if (ALIGN_DOWN(addr, CONT_PMD_SIZE) == addr) > + return false; > + > + pmdp = pmd_offset(pudp, addr); > + pmd = pmdp_get(pmdp); > + if (!pmd_present(pmd)) > + return false; > + > + if (pmd_cont(pmd)) > + return true; > + > + /* > + * PMD level: > + * > + * If addr is PMD_SIZE aligned - already on a leaf boundary > + */ > + if (ALIGN_DOWN(addr, PMD_SIZE) == addr) > + return false; > + > + if (pmd_leaf(pmd)) > + return true; > + > + /* > + * CONT_PTE level: > + * > + * If addr is CONT_PTE_SIZE aligned - already on a leaf boundary > + */ > + if (ALIGN_DOWN(addr, CONT_PTE_SIZE) == addr) > + return false; > + > + ptep = pte_offset_kernel(pmdp, addr); > + pte = __ptep_get(ptep); > + if (!pte_present(pte)) > + return false; > + > + if (pte_cont(pte)) > + return true; > + > + /* > + * PTE level: > + * > + * If addr is PAGE_SIZE aligned - already on a leaf boundary > + */ > + if (ALIGN_DOWN(addr, PAGE_SIZE) == addr) > + return false; > + return true; return !IS_ALIGNED(addr, PAGE_SIZE); > +} > + > +static bool can_unmap_without_split(unsigned long pfn, unsigned long nr_pages) > +{ > + unsigned long phys_start, phys_end, size, start, end; > + > + phys_start = PFN_PHYS(pfn); > + phys_end = phys_start + nr_pages * PAGE_SIZE; > + > + /* > + * PFN range's linear map edges are leaf entry aligned > + */ > + start = __phys_to_virt(phys_start); > + end = __phys_to_virt(phys_end); > + if (addr_splits_kernel_leaf(start) || addr_splits_kernel_leaf(end)) { > + pr_warn("[%lx %lx] splits a leaf entry in linear map\n", > + phys_start, phys_end); > + return false; > + } > + > + /* > + * PFN range's vmemmap edges are leaf entry aligned > + */ > + size = nr_pages * sizeof(struct page); > + start = (unsigned long)pfn_to_page(pfn); > + end = start + size; As arm64 cannot be used with CONFIG_SPARSEMEM (only with CONFIG_SPARSEMEM_VMEMMAP, as it sets SPARSEMEM_VMEMMAP_ENABLE), I think you can just do start = (unsigned long)pfn_to_page(pfn); end = (unsigned long)pfn_to_page(pfn + nr_pages); As pfn_to_page() is just simple calculation that works even if "pfn+nr_pages" does not exist. > + if (addr_splits_kernel_leaf(start) || addr_splits_kernel_leaf(end)) { > + pr_warn("[%lx %lx] splits a leaf entry in vmemmap\n", > + phys_start, phys_end); > + return false; > + } > + return true; > +} > + -- Cheers, David