linux-kernel - [PATCH 1/3] LoongArch: Prefer top-down allocation after arch_mem

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Message-ID: <20260127084619.1140692-1-chenhuacai@loongson.cn>
Date: Tue, 27 Jan 2026 16:46:17 +0800
From: Huacai Chen <chenhuacai@...ngson.cn>
To: Huacai Chen <chenhuacai@...nel.org>
Cc: loongarch@...ts.linux.dev,
	Xuefeng Li <lixuefeng@...ngson.cn>,
	Guo Ren <guoren@...nel.org>,
	Xuerui Wang <kernel@...0n.name>,
	Jiaxun Yang <jiaxun.yang@...goat.com>,
	linux-kernel@...r.kernel.org,
	Huacai Chen <chenhuacai@...ngson.cn>,
	stable@...r.kernel.org
Subject: [PATCH 1/3] LoongArch: Prefer top-down allocation after arch_mem_init()

Currently we use bottom-up allocation after sparse_init(), the reason is
sparse_init() need a lot of memory, and bottom-up allocation may exhaust
precious low memory (below 4GB). On the other hand, SWIOTLB and CMA need
low memories for DMA32, so swiotlb_init() and dma_contiguous_reserve()
need bottom-up allocation.

Since swiotlb_init() and dma_contiguous_reserve() are both called in
arch_mem_init(), we no longer need bottom-up allocation after that. So
we set the allocation policy to top-down at the end of arch_mem_init(),
in order to avoid later memory allocations (such as KASAN) exhaust low
memory.

This solve at least two problems:
1. Some buggy BIOSes use 0xfd000000~0xfe000000 for secondary CPUs, but
   didn't reserve this range, which causes smpboot failures.
2. Some DMA32 devices, such as Loongson-DRM and OHCI, cannot work with
   KASAN enabled.

Cc: stable@...r.kernel.org
Signed-off-by: Huacai Chen <chenhuacai@...ngson.cn>
---
 arch/loongarch/kernel/setup.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/arch/loongarch/kernel/setup.c b/arch/loongarch/kernel/setup.c
index 20cb6f306456..2b260d15b2e2 100644
--- a/arch/loongarch/kernel/setup.c
+++ b/arch/loongarch/kernel/setup.c
@@ -421,6 +421,7 @@ static void __init arch_mem_init(char **cmdline_p)
 				   PFN_UP(__pa_symbol(&__nosave_end)));

 	memblock_dump_all();
+	memblock_set_bottom_up(false);

 	early_memtest(PFN_PHYS(ARCH_PFN_OFFSET), PFN_PHYS(max_low_pfn));
 }
-- 
2.47.3