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=1793730968; b=Q79cEr3ql+zjCsrTlrB+w7ielc4tOOJDI7i3pgBra/81d9o5GhX1rg/j2H2VBff6ELzW HFAOjM4zPzIjleFAxg1toYlDXnR2eP6l40EQvbM4gZUQ7to9duZUcUuBSttsfkccAsyAY w3hsm7yC/zL1N/bpMpYHFsgzyfSQDaTwRR7TS654KkZkb7WJQwpy0C5LCkEZodVwSFy0D RPxDE5dFHfpFesrCepow18jBUq+ukDm0MJ8bHozL8E1m5j2ROLi49AbTkLdARaaXqgA9c t08disWJIWlbbN10hx7OA7Swvimi/Z3yqVaOshS/HCPumqHgnp+UFxgfckDhyR+9kvQ== ARC-Message-Signature: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; c=relaxed/relaxed; t=1793730968; h=Received:DKIM-Signature:Date:From:To:Cc:Subject:Message-ID: References:MIME-Version:Content-Type:Content-Disposition:In-Reply-To; bh=BXtqewBsP373CPIOAi6YTJNiMe9FVQMK+bBEbUQClZU=; b=ZRGUk2AqVIxHnKzW9F+WlVqIWewYS4AmfhE1gyyGQ+e4GoEpZ5MLIXegafmcNFVAIi7z Ix1W9EUBKURvdqIX5HA1BMUYA71BgfrZEiwfl2fxwbVnvWW36KDn5F2Hqm8idTdA+PZT5 aslgRpyLGY84pqCL1C3wrsD49iSdFPBew/Ih47W0i701oMUFV9vXaS+kOLhknSGVjhEMk xZyI0Cj7PcuaWYsVeBo3dE6O9Zxt3Ut+CqdT935SJ7hZrZFhLIvgqs7+bTapuoE0XUlUj n93+N0Xb+muN886QmewExo1d/KQyqXm2TM5345UrJ1UvKnevqtnZakLzyeyrVK6zXGA== 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=UH8BzPZP; 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 89C7C606A32E for ; Tue, 3 Nov 2026 18:36:08 +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=UH8BzPZP Received: by smtp.kernel.org (Postfix) with ESMTPSA id F16B61F00A15; Tue, 3 Nov 2026 18:36:04 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1793730968; bh=BXtqewBsP373CPIOAi6YTJNiMe9FVQMK+bBEbUQClZU=; h=Date:From:To:Cc:Subject:References:In-Reply-To; b=UH8BzPZP8cqAcgbW0cJ2L2+EeGFa8v1ugJxgyujaVpJRuRY+sSCplVUE39iUvpYeu D6ecFGlEC6/3TCkletVwNohOQv2BJNrNjwk7jIgifq72h/eagD0XCTcNzHg9FYs0Wj HlsU+MQghxeeGnDEUZNxmhKoSDPp8ER3jnt71KYlUs8BJsH3yp3I56jFVTzAPJDi3T cI/VO1uVPhFX8+5NWFOi4/0yk7U/i8sJgZkThrJDsHx98cnlkrRyhWZk+pz6MJC5lV ovSEHcqvAE49zuIHSkwCpalvys7U15hZlL6DBvwTR10SnI2vHvMkF3Nfb27NjxX9ow DLArzvkvQGnTg== Date: Tue, 3 Nov 2026 20:36:00 +0200 From: Mike Rapoport To: Pratyush Yadav Cc: 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 Message-ID: References: <20261103-kho-bootmem-sizing-v1-0-45aa74098f34@kernel.org> <20261103-kho-bootmem-sizing-v1-2-45aa74098f34@kernel.org> <2vxz5wydnbuj.fsf@kernel.org> 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; charset=us-ascii Content-Disposition: inline In-Reply-To: <2vxz5wydnbuj.fsf@kernel.org> 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? > 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. > -- > Regards, > Pratyush Yadav -- Sincerely yours, Mike.