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 1855C41B344; Mon, 2 Mar 2026 15:28:14 +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=1772465295; cv=none; b=bLS8Coa9Moqp4zztd3la/5TIqyMNXcilFrVxDnDELxYjk8TfZPMItbJly8Be2acyeeJdHSflkm5ooOgZmpyoZkDm4+XGqUPVVRhmHhC/fvHmOSP6qp2Y8DsB2bxZQrAhPGUt9VckbsnKpqTZ6grLYGc1f1HMF3R/apsBwM2HWYg= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1772465295; c=relaxed/simple; bh=bLBAyZ1BexHi7dQN3dUIva97z4hRfah1yjE2p8lfp+4=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=IVRilyn0SzEgm5/L0Y5Tgct3tnBPYn+rfGu1sQhLzQthziDPTlWppQ084udInkoV9wlIt2xxFtT4wKh9jcU/g6lfd+ySfj8tTgchfpncascl4F5rMHR2lwf3tRj1hdZZPZgmX2YNFgad7owLKkzPjBcJHZTOqQCp/aH5OgOytvs= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=r9gCK//1; 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="r9gCK//1" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 7D9B8C19423; Mon, 2 Mar 2026 15:28:11 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1772465294; bh=bLBAyZ1BexHi7dQN3dUIva97z4hRfah1yjE2p8lfp+4=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=r9gCK//1M15CcqoJAY0n92aVVUqokthG4UqgAiNM62ZoGveq2ft2l5TFdox26xFxq CZQuG6r5pD2FJWHlojAbgGBDZPti65Jh7rmtO0vgqYK3Kqnk/8+dQxmE1dGbvhRyk5 663Ez/MG1esVyeJQePMh3Uz7RF7aX/LLxPETOl2ZhZFYpNWSRKZZdQjECNg8iBEKVF XHMUgNg6H04oyxT4o1hiP/5IQzFQ+MYtsjue1PF50rvTWB87FIcBjtYzw4S3LvHnNM WXJpFkAFTNDm3WvQ6lZDaH6B0HkhqdSlE5HlZQVy2EVUOrdkttjpYBJdJmUFMeLXsw TvDFQjadOYPjA== Message-ID: Date: Mon, 2 Mar 2026 16:28:08 +0100 Precedence: bulk X-Mailing-List: stable@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH V3 1/2] arm64/mm: Enable batched TLB flush in unmap_hotplug_range() 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, stable@vger.kernel.org References: <20260224062423.972404-1-anshuman.khandual@arm.com> <20260224062423.972404-2-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: > During a memory hot remove operation, both linear and vmemmap mappings for > the memory range being removed, get unmapped via unmap_hotplug_range() but > mapped pages get freed only for vmemmap mapping. This is just a sequential > operation where each table entry gets cleared, followed by a leaf specific > TLB flush, and then followed by memory free operation when applicable. > > This approach was simple and uniform both for vmemmap and linear mappings. > But linear mapping might contain CONT marked block memory where it becomes > necessary to first clear out all entire in the range before a TLB flush. > This is as per the architecture requirement. Hence batch all TLB flushes > during the table tear down walk and finally do it in unmap_hotplug_range(). > > Prior to this fix, it was hypothetically possible for a speculative access > to a higher address in the contiguous block to fill the TLB with shattered > entries for the entire contiguous range after a lower address had already > been cleared and invalidated. Due to the table entries being shattered, the > subsequent TLB invalidation for the higher address would not then clear the > TLB entries for the lower address, meaning stale TLB entries could persist. > > Besides it also helps in improving the performance via TLBI range operation > along with reduced synchronization instructions. The time spent executing > unmap_hotplug_range() improved 97% measured over a 2GB memory hot removal > in KVM guest. > > This scheme is not applicable during vmemmap mapping tear down where memory > needs to be freed and hence a TLB flush is required after clearing out page > table entry. > > 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") > Cc: stable@vger.kernel.org > Reviewed-by: Ryan Roberts > Signed-off-by: Ryan Roberts > Signed-off-by: Anshuman Khandual > --- > arch/arm64/mm/mmu.c | 81 +++++++++++++++++++++++++++++++++++++-------- > 1 file changed, 67 insertions(+), 14 deletions(-) > > diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c > index a6a00accf4f9..dfb61d218579 100644 > --- a/arch/arm64/mm/mmu.c > +++ b/arch/arm64/mm/mmu.c > @@ -1458,10 +1458,32 @@ static void unmap_hotplug_pte_range(pmd_t *pmdp, unsigned long addr, > > WARN_ON(!pte_present(pte)); > __pte_clear(&init_mm, addr, ptep); > - flush_tlb_kernel_range(addr, addr + PAGE_SIZE); > - if (free_mapped) > + if (free_mapped) { > + /* > + * If page is part of an existing contiguous > + * memory block, individual TLB invalidation > + * here would not be appropriate. Instead it > + * will require clearing all entries for the > + * memory block and subsequently a TLB flush > + * for the entire range. > + */ I'm not sure about repeating these longish comments a couple of times :) For example, I think you can drop the ones regarding the TLB flush ("TLB flush is batched in unmap_hotplug_range ...") completely. unmap_hotplug_range(), the only caller, is pretty clear about that. And anybody reading that code should be able to spot the "!free_mapped" case easily. Alternatively, just say /* unmap_hotplug_range() flushes TLB for !free_mapped */ "TLB flush is essential for freeing memory." is rather obvious when freeing memory, so I would drop that as well. Regarding pte_cont(), can we shorten that to /* CONT blocks in the vmemmap are not supported. */ Anybody who wants to figure *why* can lookup your patch where you add that comment+check. I did not check whether people suggested to add these comments in previous versions. But to me they don't add a lot of real value that couldn't be had from the code already (or common sense: freeing requires prior TLB flush). > + WARN_ON(pte_cont(pte)); > + > + /* > + * TLB flush is essential for freeing memory. > + */ > + flush_tlb_kernel_range(addr, addr + PAGE_SIZE); > free_hotplug_page_range(pte_page(pte), > PAGE_SIZE, altmap); > + } [...] > WARN_ON(!pud_table(pud)); > @@ -1553,6 +1597,7 @@ static void unmap_hotplug_p4d_range(pgd_t *pgdp, unsigned long addr, > static void unmap_hotplug_range(unsigned long addr, unsigned long end, > bool free_mapped, struct vmem_altmap *altmap) > { > + unsigned long start = addr; > unsigned long next; > pgd_t *pgdp, pgd; > > @@ -1574,6 +1619,14 @@ static void unmap_hotplug_range(unsigned long addr, unsigned long end, > WARN_ON(!pgd_present(pgd)); > unmap_hotplug_p4d_range(pgdp, addr, next, free_mapped, altmap); > } while (addr = next, addr < end); > + > + /* > + * Batched TLB flush only for linear mapping which > + * might contain CONT blocks, and does not require > + * freeing up memory as well. > + */ Also, here, I don't think the comment really adds value. * !free_mapped -> linear mapping, no freeing of memory * CONT blocks -> irrelevant, you can batch in either case > + if (!free_mapped) > + flush_tlb_kernel_range(start, end); > } -- Cheers, David