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

[Kiryl Shutsemau <kirill@shutemov.name>]

On Wed, Sep 24, 2025 at 12:48:53AM +0000, 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>
> 
> ---
> 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);
> +			}

Indent level gets extreme.

Any reason not to fold slot check inside mm_slot_entry() as I suggested
on early version.

>  		}
>  	}
>  	spin_unlock(&ksm_mmlist_lock);
> -- 
> 2.34.1
> 

-- 
  Kiryl Shutsemau / Kirill A. Shutemov