From mboxrd@z Thu Jan 1 00:00:00 1970 Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.remote-ip=100.103.45.18 ARC-Seal: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; cv=none; t=1793794409; b=dH8EGbTxb7G3Dq6SO+xxZJAHxmJx50C9fY7zyfZnxRYZ5acKmYNHEiOf+9DWwSIO3dxI kfBrC646jVzE/SA3uSDjG0QbQaXoELA4d5ye+PyDgEL2mGropbF4lofqbvifkpWB8B5Cz /W6bl4308d9HqX//o8ufw6wSARm5wuNCj2X/PPuviF7Q5fCj13TpMYZ/quhr598xG6RGH CNXtmNATsqijyxB+ioMUHGVC0y8qo+FL8rGSO68dBbdd+XsO3LX+EtvnDTQin+NwytSoG uLnuUWWF/pCOb181dEVPrCevqSo1AqL1drTC+4ZpvYHLHT1W7/raN6INbxeQc3My0Cg== ARC-Message-Signature: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; c=relaxed/relaxed; t=1793794409; h=Received:DKIM-Signature:From:To:Cc:Subject:In-Reply-To:References: Date:Message-ID:User-Agent:MIME-Version:Content-Type; bh=xI1Euu70YUflyL8kMW3VkLtf2BBOiDl/YP7uIS54om4=; b=Wt1uU1hes96VXVTtbWgFAYFcW1bRs7YOXlNSGTyenvrFO94XkLm0edK3OVXuLSO8Gn1p VjiXWiyUPmh3uMH2d6HEqTShSqg4KJi5XObOr6dD5anApWAyYwLdEpE6FTr86EgzJh9K1 KKeqyP9ViOAeQk/ts7WZJyt25bhXVNgi0B0DP3mJe8wBGUcO8zyb9wjcUUctHAkqC+U9M kPjLCsfpDTHPoRt3W1PMkQr2yvSM2g2TprigsC+P2bC1hDTJI3+mGa9dUYHAPqqLyninD 3qJGHKT85VLVNK80Xuyd4jW1xziXh2/2k3LxRwgR1OwltKupcNSS7TcT5dAgv3KqEYg== ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dkim=pass header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=bjt0GwCP; arc=none smtp.remote-ip=100.103.45.18 Received: from smtp.kernel.org (quasi.space.kernel.org [100.103.45.18]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (prime256v1)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 871DC606A326 for ; Wed, 4 Nov 2026 12:13:29 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key, unprotected) header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=bjt0GwCP Received: by smtp.kernel.org (Postfix) with ESMTPSA id 0B4871F000C1; Wed, 4 Nov 2026 12:13:26 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1793794409; bh=xI1Euu70YUflyL8kMW3VkLtf2BBOiDl/YP7uIS54om4=; h=From:To:Cc:Subject:In-Reply-To:References:Date; b=bjt0GwCPsVqSBZmBajC5cqnTxurvBTha5TBGNnuGjMcY/0RTazrh5fIHq8bFWGQJZ Sl3ZZM5n6doE/Ftx4asgxVmHQR2R1O/dLRdMTU+uCRc5KobE6A61lhMFJxz2AAXL/n mwLfVnHy8mP8qKSgleVvE0YCsvOODm/0zkLprwAHYenBDBa5Hu96BT20L6ne2x8mqc eaMc0NaaRIaVF4HKwPhtv0exAzuTDF+YQSqiLvhEhZtocQ5lU6tno7VrHd/Ukf5JPF lv8lFVhK9j8reWbzfJSRVa1gx4MjwWy8dUqz65qOkXdM62ndm/VwxfOTM4FL5S4tPK yqE/KXNjxSACw== From: Pratyush Yadav To: Mike Rapoport Cc: Pratyush Yadav , Pasha Tatashin , Alexander Graf , Andrew Morton , Chen Pei , George Guo , Kexin Liu , Sourabh Jain , kexec@lists.infradead.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org Subject: Re: [PATCH 2/2] kho: fix bootmem sizing In-Reply-To: (Mike Rapoport's message of "Tue, 3 Nov 2026 20:36:00 +0200") References: <20261103-kho-bootmem-sizing-v1-0-45aa74098f34@kernel.org> <20261103-kho-bootmem-sizing-v1-2-45aa74098f34@kernel.org> <2vxz5wydnbuj.fsf@kernel.org> Date: Wed, 04 Nov 2026 13:13:25 +0100 Message-ID: <2vxzse1glrq2.fsf@kernel.org> User-Agent: Gnus/5.13 (Gnus v5.13) X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 Content-Type: text/plain On Tue, Nov 03 2026, Mike Rapoport wrote: > On Tue, Nov 03, 2026 at 05:01:08PM +0100, Pratyush Yadav wrote: >> Hi Mike, >> >> On Tue, Nov 03 2026, Mike Rapoport (Microsoft) wrote: >> >> > Bootmem areas are sized from overlapping views of the memory reserved in >> > memblock: >> > >> > * the global area is sized from all reserved memory, including memory >> > allocated on specific nodes, so on NUMA systems everything allocated >> > on a node is accounted in both the global area and the area of that >> > node. >> > * per-node sizes are calculated after the lowmem and the global areas >> > are reserved. With CONFIG_NUMA=n all memblock regions belong to node 0, >> > so the lowmem and the global areas are accounted in the size of node 0 >> > area and scaled once again. >> > >> > As a result, bootmem is about twice bigger than it should be on NUMA >> > systems and about four times bigger with CONFIG_NUMA=n. On small systems >> > reservation of the bootmem may fail and then KHO gets disabled. >> > >> > Make the areas cover separate parts of the reserved memory: the lowmem >> > area covers allocations below ARCH_LOW_ADDRESS_LIMIT, per-node areas >> > cover allocations above it on each node and the global area covers >> > allocations that are not bound to any node. Calculate all sizes before >> > reserving any of the areas. >> > >> > With CONFIG_NUMA=n the global area becomes empty and it is not reserved. >> > >> > Bootmem sizes in MiB in x86 QEMU VMs, before -> after: >> > >> > CONFIG_NUMA=y CONFIG_NUMA=n >> > 1G, 1 node 176 -> 92 320 -> 80 >> > 4G, 1 node 508 -> 332 1264 -> 316 >> > 16G, 2 nodes 1268 -> 704 2944 -> 736 >> > 32G, 4 nodes 2296 -> 1224 5196 -> 1300 >> > 64G, 4 nodes 4344 -> 2248 9664 -> 2420 >> > >> > Fixes: 3dc92c311498 ("kexec: add Kexec HandOver (KHO) generation helpers") >> > Reported-by: Kexin Liu >> > Reported-by: George Guo >> > Closes: https://lore.kernel.org/all/20260904025101.9959-1-dongtai.guo@linux.dev >> > Reported-by: Sourabh Jain >> > Closes: https://lore.kernel.org/all/20260922131217.698809-1-sourabhjain@linux.ibm.com >> > Signed-off-by: Mike Rapoport (Microsoft) >> >> Thanks for the patch! My LLM caught an interesting problems that is >> worth considering. >> >> On arm64, ARCH_LOW_ADDRESS_LIMIT is arm64_dma_phys_limit - 1, and >> dma_limits_init() sets the default value to PHYS_MASK + 1, and it can >> also be memblock_end_of_dram() in some cases. So on arm64, it is >> possible for lowmem to span the entire RAM. >> >> Currently, that results in over-allocation of bootmem, and it ends up >> being 4x what it should be. While obviously problematic, it should still >> work. With this patch, all memory is in lowmem, and nothing ever goes to >> per-node bootmem (except for the node that hosts the lowmem area). >> >> All this is from reading the code, I haven't tried anything out yet. >> >> Not sure how to fix this though. Should we perhaps use >> min(ARCH_LOW_ADDRESS_LIMIT, U32_MAX) for the lowmem limit? > > Argh :) > > Maybe we reset lowmem to zero if memblock_end_of_DRAM() < > ARCH_LOW_ADDRESS_LIMIT? Sounds good. > >> Also, a couple nitpicks below. >> >> > --- >> > kernel/liveupdate/kexec_handover.c | 83 ++++++++++++++++++++------------------ >> > 1 file changed, 44 insertions(+), 39 deletions(-) >> > >> > + >> > + lowmem = reserved_mem_size(ARCH_LOW_ADDRESS_LIMIT, NUMA_NO_NODE); >> > + global = reserved_mem_size(MEMBLOCK_ALLOC_ANYWHERE, NUMA_NO_NODE); >> > + global -= lowmem; >> >> Nit: Please add a short comment above this to explain why you remove >> lowmem from global. > > I prefer to extend the comment above the function to say that calculations > for global and node that includes lowmem remove that. Sure, that works too. -- Regards, Pratyush Yadav