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

[Pasha Tatashin]

On 05-09 10:44, George Guo wrote:
> From: George Guo <guodongtai@xxxxxxxxxx>
> 
> 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. LoongArch), this gives a depth of 4,
> with the top-level shift at bit 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.
> 
> Fixes: 3f2ad90060f6 ("kho: adopt radix tree for preserved memory tracking")
> 
> Tested-by: Kexin Liu <liukexin@xxxxxxxxxx>
> Signed-off-by: George Guo <guodongtai@xxxxxxxxxx>

Reviewed-by: Pasha Tatashin <pasha.tatashin@xxxxxxxxxx> 

The patch fixes 16K support on ARM64.

Pasha