Re: [PATCH 1/1] kho: fix KHO_TREE_MAX_DEPTH for non-4KB page sizes

[George Guo <dongtai.guo@linux.dev>]

Sorry for the late reply.

On Mon, May 11 2026, Pratyush Yadav wrote:

> As of now, we only support KHO on x86 and arm64. Are you working on
> supporting it for LoongArch? What is your use case?

Yes, we are adding KHO support for LoongArch (16KB page size). The
LoongArch patches are being prepared separately. This fix is a
prerequisite.

> Maybe I don't understand the math so well, but I can't see the problem.
> ...
> level = 4, s = 50, idx = 1
> What am I missing?

The issue is that all three operations start the traversal at
KHO_TREE_MAX_DEPTH - 1, not KHO_TREE_MAX_DEPTH:

  kho_radix_add_page()  line 180: for (i = KHO_TREE_MAX_DEPTH - 1; i > 0; i--)
  kho_radix_del_page()  line 255: for (i = KHO_TREE_MAX_DEPTH - 1; i > 0; i--)
  kho_radix_walk_tree() line 356: __kho_radix_walk_tree(..., KHO_TREE_MAX_DEPTH - 1, ...)

So with depth=4 the effective top level is 3, not 4.

phys_to_page() of that address faults in kho_preserved_memory_reserve().

This is confirmed by a kernel panic on 7.1-rc3 LoongArch (16KB pages)
without the fix. With depth=5 the top level is 4 (shift=50):

  (key >> 50) % 2048 = 1   /* order bit correctly captured */

Signed-off-by: George Guo <guodongtai@kylinos.cn>

Panic log on 7.1-rc3 LoongArch (16KB pages) without the fix:

[    0.000000] CPU 0 Unable to handle kernel paging request at virtual address 00003d3ffe000028, era == 90000000c162f10c, ra == 90000000c162f0f8
[    0.000000] Oops[#1]:
...
[    0.000000] Call Trace:
[    0.000000] [<90000000c162f10c>] kho_preserved_memory_reserve+0xc4/0xe8
[    0.000000] [<90000000c0129f88>] __kho_radix_walk_tree+0xf0/0x138
[    0.000000] [<90000000c0129f10>] __kho_radix_walk_tree+0x78/0x138
[    0.000000] [<90000000c012b730>] kho_radix_walk_tree+0x88/0xe8
[    0.000000] [<90000000c162f874>] kho_memory_init+0x220/0x4e4
[    0.000000] [<90000000c1639b38>] mm_core_init+0x168/0x1a0
[    0.000000] [<90000000c1620d50>] start_kernel+0x5c4/0x778
[    0.000000] Kernel panic - not syncing: Attempted to kill the idle task!