Re: [Patch v4 2/2] mm/khugepaged: remove definition of struct khugepaged_mm_slot

[Muhammad Usama Anjum <MUsamaAnjum@gmail.com>]

On 9/27/25 5:45 AM, Wei Yang wrote:
> Current code calls mm_slot_entry() even when we don't have a valid slot,
> which is not future proof. Currently, this is not a problem because
> "slot" is the first member in struct khugepaged_mm_slot.
> 
> While struct khugepaged_mm_slot is just a wrapper of struct mm_slot, there
> is no need to define it.
> 
> Remove the definition of struct khugepaged_mm_slot, so there is not chance
> to miss use mm_slot_entry().
> 
> Signed-off-by: Wei Yang <richard.weiyang@gmail.com>
> Acked-by: Lance Yang <lance.yang@linux.dev>
> Reviewed-by: Dev Jain <dev.jain@arm.com>
> Cc: Lance Yang <lance.yang@linux.dev>
> Cc: David Hildenbrand <david@redhat.com>
> Cc: Dev Jain <dev.jain@arm.com>
> Cc: Kiryl Shutsemau <kirill@shutemov.name>
> Cc: xu xin <xu.xin16@zte.com.cn>
> Cc: SeongJae Park <sj@kernel.org>
> Cc: Nico Pache <npache@redhat.com>
Acked-by: Muhammad Usama Anjum <usama.anjum@collabora.com>

> 
> ---
> v3:
>   * adjust changelog
>   * rename the slab cache to "mm_slot"
> v2:
>   * fix a PF reported by SeongJae, where slot is changed to next one
> ---
>  mm/khugepaged.c | 55 ++++++++++++++++---------------------------------
>  1 file changed, 18 insertions(+), 37 deletions(-)
> 
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index 204ce3059267..67540078083b 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -103,14 +103,6 @@ struct collapse_control {
>  	nodemask_t alloc_nmask;
>  };
>  
> -/**
> - * struct khugepaged_mm_slot - khugepaged information per mm that is being scanned
> - * @slot: hash lookup from mm to mm_slot
> - */
> -struct khugepaged_mm_slot {
> -	struct mm_slot slot;
> -};
> -
>  /**
>   * struct khugepaged_scan - cursor for scanning
>   * @mm_head: the head of the mm list to scan
> @@ -121,7 +113,7 @@ struct khugepaged_mm_slot {
>   */
>  struct khugepaged_scan {
>  	struct list_head mm_head;
> -	struct khugepaged_mm_slot *mm_slot;
> +	struct mm_slot *mm_slot;
>  	unsigned long address;
>  };
>  
> @@ -384,7 +376,7 @@ int hugepage_madvise(struct vm_area_struct *vma,
>  
>  int __init khugepaged_init(void)
>  {
> -	mm_slot_cache = KMEM_CACHE(khugepaged_mm_slot, 0);
> +	mm_slot_cache = KMEM_CACHE(mm_slot, 0);
>  	if (!mm_slot_cache)
>  		return -ENOMEM;
>  
> @@ -438,7 +430,6 @@ static bool hugepage_pmd_enabled(void)
>  
>  void __khugepaged_enter(struct mm_struct *mm)
>  {
> -	struct khugepaged_mm_slot *mm_slot;
>  	struct mm_slot *slot;
>  	int wakeup;
>  
> @@ -447,12 +438,10 @@ void __khugepaged_enter(struct mm_struct *mm)
>  	if (unlikely(mm_flags_test_and_set(MMF_VM_HUGEPAGE, mm)))
>  		return;
>  
> -	mm_slot = mm_slot_alloc(mm_slot_cache);
> -	if (!mm_slot)
> +	slot = mm_slot_alloc(mm_slot_cache);
> +	if (!slot)
>  		return;
>  
> -	slot = &mm_slot->slot;
> -
>  	spin_lock(&khugepaged_mm_lock);
>  	mm_slot_insert(mm_slots_hash, mm, slot);
>  	/*
> @@ -480,14 +469,12 @@ void khugepaged_enter_vma(struct vm_area_struct *vma,
>  
>  void __khugepaged_exit(struct mm_struct *mm)
>  {
> -	struct khugepaged_mm_slot *mm_slot;
>  	struct mm_slot *slot;
>  	int free = 0;
>  
>  	spin_lock(&khugepaged_mm_lock);
>  	slot = mm_slot_lookup(mm_slots_hash, mm);
> -	mm_slot = mm_slot_entry(slot, struct khugepaged_mm_slot, slot);
> -	if (mm_slot && khugepaged_scan.mm_slot != mm_slot) {
> +	if (slot && khugepaged_scan.mm_slot != slot) {
>  		hash_del(&slot->hash);
>  		list_del(&slot->mm_node);
>  		free = 1;
> @@ -496,9 +483,9 @@ void __khugepaged_exit(struct mm_struct *mm)
>  
>  	if (free) {
>  		mm_flags_clear(MMF_VM_HUGEPAGE, mm);
> -		mm_slot_free(mm_slot_cache, mm_slot);
> +		mm_slot_free(mm_slot_cache, slot);
>  		mmdrop(mm);
> -	} else if (mm_slot) {
> +	} else if (slot) {
>  		/*
>  		 * This is required to serialize against
>  		 * hpage_collapse_test_exit() (which is guaranteed to run
> @@ -1432,9 +1419,8 @@ static int hpage_collapse_scan_pmd(struct mm_struct *mm,
>  	return result;
>  }
>  
> -static void collect_mm_slot(struct khugepaged_mm_slot *mm_slot)
> +static void collect_mm_slot(struct mm_slot *slot)
>  {
> -	struct mm_slot *slot = &mm_slot->slot;
>  	struct mm_struct *mm = slot->mm;
>  
>  	lockdep_assert_held(&khugepaged_mm_lock);
> @@ -1451,7 +1437,7 @@ static void collect_mm_slot(struct khugepaged_mm_slot *mm_slot)
>  		 */
>  
>  		/* khugepaged_mm_lock actually not necessary for the below */
> -		mm_slot_free(mm_slot_cache, mm_slot);
> +		mm_slot_free(mm_slot_cache, slot);
>  		mmdrop(mm);
>  	}
>  }
> @@ -2394,7 +2380,6 @@ static unsigned int khugepaged_scan_mm_slot(unsigned int pages, int *result,
>  	__acquires(&khugepaged_mm_lock)
>  {
>  	struct vma_iterator vmi;
> -	struct khugepaged_mm_slot *mm_slot;
>  	struct mm_slot *slot;
>  	struct mm_struct *mm;
>  	struct vm_area_struct *vma;
> @@ -2405,14 +2390,12 @@ static unsigned int khugepaged_scan_mm_slot(unsigned int pages, int *result,
>  	*result = SCAN_FAIL;
>  
>  	if (khugepaged_scan.mm_slot) {
> -		mm_slot = khugepaged_scan.mm_slot;
> -		slot = &mm_slot->slot;
> +		slot = khugepaged_scan.mm_slot;
>  	} else {
>  		slot = list_first_entry(&khugepaged_scan.mm_head,
>  				     struct mm_slot, mm_node);
> -		mm_slot = mm_slot_entry(slot, struct khugepaged_mm_slot, slot);
>  		khugepaged_scan.address = 0;
> -		khugepaged_scan.mm_slot = mm_slot;
> +		khugepaged_scan.mm_slot = slot;
>  	}
>  	spin_unlock(&khugepaged_mm_lock);
>  
> @@ -2510,7 +2493,7 @@ static unsigned int khugepaged_scan_mm_slot(unsigned int pages, int *result,
>  breakouterloop_mmap_lock:
>  
>  	spin_lock(&khugepaged_mm_lock);
> -	VM_BUG_ON(khugepaged_scan.mm_slot != mm_slot);
> +	VM_BUG_ON(khugepaged_scan.mm_slot != slot);
>  	/*
>  	 * Release the current mm_slot if this mm is about to die, or
>  	 * if we scanned all vmas of this mm.
> @@ -2522,16 +2505,14 @@ static unsigned int khugepaged_scan_mm_slot(unsigned int pages, int *result,
>  		 * mm_slot not pointing to the exiting mm.
>  		 */
>  		if (!list_is_last(&slot->mm_node, &khugepaged_scan.mm_head)) {
> -			slot = list_next_entry(slot, mm_node);
> -			khugepaged_scan.mm_slot =
> -				mm_slot_entry(slot, struct khugepaged_mm_slot, slot);
> +			khugepaged_scan.mm_slot = list_next_entry(slot, mm_node);
>  			khugepaged_scan.address = 0;
>  		} else {
>  			khugepaged_scan.mm_slot = NULL;
>  			khugepaged_full_scans++;
>  		}
>  
> -		collect_mm_slot(mm_slot);
> +		collect_mm_slot(slot);
>  	}
>  
>  	return progress;
> @@ -2618,7 +2599,7 @@ static void khugepaged_wait_work(void)
>  
>  static int khugepaged(void *none)
>  {
> -	struct khugepaged_mm_slot *mm_slot;
> +	struct mm_slot *slot;
>  
>  	set_freezable();
>  	set_user_nice(current, MAX_NICE);
> @@ -2629,10 +2610,10 @@ static int khugepaged(void *none)
>  	}
>  
>  	spin_lock(&khugepaged_mm_lock);
> -	mm_slot = khugepaged_scan.mm_slot;
> +	slot = khugepaged_scan.mm_slot;
>  	khugepaged_scan.mm_slot = NULL;
> -	if (mm_slot)
> -		collect_mm_slot(mm_slot);
> +	if (slot)
> +		collect_mm_slot(slot);
>  	spin_unlock(&khugepaged_mm_lock);
>  	return 0;
>  }