Re: [PATCH] mm/sparse: Fix flags overlap in section_mem_map

[David Hildenbrand]

On 27.04.21 10:30, Wang Wensheng wrote:
> The section_mem_map member of struct mem_section stores some flags and
> the address of struct page array of the mem_section.
>
> Additionally the node id of the mem_section is stored during early boot,
> where the struct page array has not been allocated. In other words, the
> higher bits of section_mem_map are used for two purpose, and the node id
> should be clear properly after the early boot.
>
> Currently the node id field is overlapped with the flag field and cannot
> be clear properly. That overlapped bits would then be treated as
> mem_section flags and may lead to unexpected side effects.
>
> Define SECTION_NID_SHIFT using order_base_2 to ensure that the node id
> field always locates after flags field. That's why the overlap occurs -
> forgetting to increase SECTION_NID_SHIFT when adding new mem_section
> flag.
>
> Fixes: 326e1b8f83a4 ("mm/sparsemem: introduce a SECTION_IS_EARLY flag")
> Signed-off-by: Wang Wensheng <wangwensheng4@xxxxxxxxxx>
> ---
>   include/linux/mmzone.h | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> index 47946ce..b01694d 100644
> --- a/include/linux/mmzone.h
> +++ b/include/linux/mmzone.h
> @@ -1325,7 +1325,7 @@ extern size_t mem_section_usage_size(void);
>   #define SECTION_TAINT_ZONE_DEVICE	(1UL<<4)
>   #define SECTION_MAP_LAST_BIT		(1UL<<5)
>   #define SECTION_MAP_MASK		(~(SECTION_MAP_LAST_BIT-1))
> -#define SECTION_NID_SHIFT		3
> +#define SECTION_NID_SHIFT		order_base_2(SECTION_MAP_LAST_BIT)
>   
>   static inline struct page *__section_mem_map_addr(struct mem_section *section)
>   {

Well, all sections around during boot that have an early NID are early 
... so it's not an issue with SECTION_IS_EARLY, no? I mean, it's ugly, 
but not broken.

But it's an issue with SECTION_TAINT_ZONE_DEVICE, AFAIKT. 
sparse_init_one_section() would leave the bit set if the nid happens to 
have that bit set (e.g., node 2,3). It's semi-broken then, because we 
force all pfn_to_online_page() through the slow path.


That whole section flag setting code is fragile.

--
Thanks,

David / dhildenb