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KHO_TREE_MAX_DEPTH is calculated as:
DIV_ROUND_UP(KHO_ORDER_0_LOG2 - KHO_BITMAP_SIZE_LOG2,
KHO_TABLE_SIZE_LOG2) + 1
For systems with 16KB pages (e.g. arm64 with CONFIG_ARM64_16K_PAGES=y or
LoongArch), this gives a depth of 4. Since levels are 0 based, with
depth = 4 the effective top level is 3 and the top-level shift at bit 39.
PAGE_SHIFT = 14
KHO_BITMAP_SIZE_LOG2 = PAGE_SHIFT + 3 = 17
KHO_TABLE_SIZE_LOG2 = log(2; (1 << PAGE_SHIFT) / 8) = 11
shift = ((3 - 1) * KHO_TABLE_SIZE_LOG2) + KHO_BITMAP_SIZE_LOG2 = 39
The order-0 bit sits at bit 50 (KHO_ORDER_0_LOG2 = 64 - PAGE_SHIFT =
50). When inserting or reading a key, the index extracted at the top
level is:
(1 << 50) >> 39 = 2048
2048 is exactly the table size (PAGE_SIZE / sizeof(phys_addr_t) = 2048
for 16KB pages), so it wraps to 0, aliasing the order bit to index 0
and losing it silently.
On the second kernel, kho_radix_decode_key() sees a key without the
order bit, calls fls64() on the wrong bit, computes a wrong order and
thus a garbage physical address. phys_to_page() of that address faults
in kho_preserved_memory_reserve(), causing a kernel panic early in boot.
Fix by adding +1 to the DIV_ROUND_UP numerator so the formula accounts
for the order bit itself, giving depth 5 for 16KB pages. The top-level
shift becomes 50, and (1 << 50) >> 50 = 1, which is nonzero and
unambiguous. For 4KB and 64KB page sizes the depth is unchanged.
Link: https://patch.msgid.link/20260509024415.33190-1-dongtai.guo@linux.dev
Fixes: 3f2ad90060 ("kho: adopt radix tree for preserved memory tracking")
Tested-by: Kexin Liu <liukexin@kylinos.cn>
Signed-off-by: George Guo <guodongtai@kylinos.cn>
Reviewed-by: Pasha Tatashin <pasha.tatashin@soleen.com>
[rppt: added actual math to the changelog]
Signed-off-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
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