Re: [Patch v3 1/2] mm/ksm: get mm_slot by mm_slot_entry() when slot is !NULL

[Chengming Zhou <chengming.zhou@linux.dev>]

On 2025/9/24 08:48, Wei Yang wrote:
> Patch series "mm_slot: fix the usage of mm_slot_entry", v2.
> 
> When using mm_slot in ksm, there is code like:
> 
>       slot = mm_slot_lookup(mm_slots_hash, mm);
>       mm_slot = mm_slot_entry(slot, struct ksm_mm_slot, slot);
>       if (mm_slot && ..) {
>       }
> 
> The mm_slot_entry() won't return a valid value if slot is NULL generally.
> But currently it works since slot is the first element of struct
> ksm_mm_slot.
> 
> To reduce the ambiguity and make it robust, access mm_slot_entry() when
> slot is !NULL.
> 
> Signed-off-by: Wei Yang <richard.weiyang@gmail.com>
> Acked-by: David Hildenbrand <david@redhat.com>
> Reviewed-by: Dev Jain <dev.jain@arm.com>
> Reviewed-by: Lance Yang <lance.yang@linux.dev>
> Cc: Kiryl Shutsemau <kirill@shutemov.name>
> Cc: xu xin <xu.xin16@zte.com.cn>
> Cc: Dan Carpenter <dan.carpenter@linaro.org>

Reviewed-by: Chengming Zhou <chengming.zhou@linux.dev>

Thanks.

> 
> ---
> v3:
>    * fix uninitialized mm_slot
> ---
>   mm/ksm.c | 22 ++++++++++++----------
>   1 file changed, 12 insertions(+), 10 deletions(-)
> 
> diff --git a/mm/ksm.c b/mm/ksm.c
> index 2dbe92e3dd52..c00a21800067 100644
> --- a/mm/ksm.c
> +++ b/mm/ksm.c
> @@ -2921,7 +2921,7 @@ int __ksm_enter(struct mm_struct *mm)
>   
>   void __ksm_exit(struct mm_struct *mm)
>   {
> -	struct ksm_mm_slot *mm_slot;
> +	struct ksm_mm_slot *mm_slot = NULL;
>   	struct mm_slot *slot;
>   	int easy_to_free = 0;
>   
> @@ -2936,15 +2936,17 @@ void __ksm_exit(struct mm_struct *mm)
>   
>   	spin_lock(&ksm_mmlist_lock);
>   	slot = mm_slot_lookup(mm_slots_hash, mm);
> -	mm_slot = mm_slot_entry(slot, struct ksm_mm_slot, slot);
> -	if (mm_slot && ksm_scan.mm_slot != mm_slot) {
> -		if (!mm_slot->rmap_list) {
> -			hash_del(&slot->hash);
> -			list_del(&slot->mm_node);
> -			easy_to_free = 1;
> -		} else {
> -			list_move(&slot->mm_node,
> -				  &ksm_scan.mm_slot->slot.mm_node);
> +	if (slot) {
> +		mm_slot = mm_slot_entry(slot, struct ksm_mm_slot, slot);
> +		if (ksm_scan.mm_slot != mm_slot) {
> +			if (!mm_slot->rmap_list) {
> +				hash_del(&slot->hash);
> +				list_del(&slot->mm_node);
> +				easy_to_free = 1;
> +			} else {
> +				list_move(&slot->mm_node,
> +					  &ksm_scan.mm_slot->slot.mm_node);
> +			}
>   		}
>   	}
>   	spin_unlock(&ksm_mmlist_lock);