Re: [PATCH 02/13] mm: extract ensure_on_mmlist() helper

[Dev Jain]

On 27/04/26 3:31 pm, Usama Arif wrote:
> When a swap entry is installed in a page table, the mm must be added
> to init_mm.mmlist so that swapoff can find and unuse its swap entries.
> This double-checked locking pattern is currently open-coded in
> try_to_unmap_one() and copy_nonpresent_pte().
> 
> Move it into ensure_on_mmlist() in mm/internal.h and convert both
> callers so it can be reused by upcoming PMD-level swap entry code
> paths that also need to register the mm with swapoff.
> 
> copy_nonpresent_pte() previously inserted into &src_mm->mmlist rather
> than &init_mm.mmlist, but the insertion point is irrelevant, mmlist
> is a circular list and swapoff walks it entirely from init_mm.mmlist,
> so only membership matters, not position.
> 
> Signed-off-by: Usama Arif <usama.arif@xxxxxxxxx>
> ---

Agree with David on the name.

The unlikely hint is dropped from the fork path. I don't think we care.

Reviewed-by: Dev Jain <dev.jain@xxxxxxx>


>  mm/internal.h | 13 +++++++++++++
>  mm/memory.c   |  9 +--------
>  mm/rmap.c     |  7 +------
>  3 files changed, 15 insertions(+), 14 deletions(-)
> 
> diff --git a/mm/internal.h b/mm/internal.h
> index 5a2ddcf68e0b..7de489689f54 100644
> --- a/mm/internal.h
> +++ b/mm/internal.h
> @@ -1952,4 +1952,17 @@ static inline int get_sysctl_max_map_count(void)
>  bool may_expand_vm(struct mm_struct *mm, const vma_flags_t *vma_flags,
>  		   unsigned long npages);
>  
> +/*
> + * Ensure @mm is on the init_mm.mmlist so swapoff can find it.
> + */
> +static inline void ensure_on_mmlist(struct mm_struct *mm)
> +{
> +	if (list_empty(&mm->mmlist)) {
> +		spin_lock(&mmlist_lock);
> +		if (list_empty(&mm->mmlist))
> +			list_add(&mm->mmlist, &init_mm.mmlist);
> +		spin_unlock(&mmlist_lock);
> +	}
> +}
> +
>  #endif	/* __MM_INTERNAL_H */
> diff --git a/mm/memory.c b/mm/memory.c
> index ea6568571131..33d7cc274e23 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -937,14 +937,7 @@ copy_nonpresent_pte(struct mm_struct *dst_mm, struct mm_struct *src_mm,
>  		if (swap_dup_entry_direct(entry) < 0)
>  			return -EIO;
>  
> -		/* make sure dst_mm is on swapoff's mmlist. */
> -		if (unlikely(list_empty(&dst_mm->mmlist))) {
> -			spin_lock(&mmlist_lock);
> -			if (list_empty(&dst_mm->mmlist))
> -				list_add(&dst_mm->mmlist,
> -						&src_mm->mmlist);
> -			spin_unlock(&mmlist_lock);
> -		}
> +		ensure_on_mmlist(dst_mm);
>  		/* Mark the swap entry as shared. */
>  		if (pte_swp_exclusive(orig_pte)) {
>  			pte = pte_swp_clear_exclusive(orig_pte);
> diff --git a/mm/rmap.c b/mm/rmap.c
> index 78b7fb5f367c..057e18cb80b0 100644
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -2302,12 +2302,7 @@ static bool try_to_unmap_one(struct folio *folio, struct vm_area_struct *vma,
>  				set_pte_at(mm, address, pvmw.pte, pteval);
>  				goto walk_abort;
>  			}
> -			if (list_empty(&mm->mmlist)) {
> -				spin_lock(&mmlist_lock);
> -				if (list_empty(&mm->mmlist))
> -					list_add(&mm->mmlist, &init_mm.mmlist);
> -				spin_unlock(&mmlist_lock);
> -			}
> +			ensure_on_mmlist(mm);
>  			dec_mm_counter(mm, MM_ANONPAGES);
>  			inc_mm_counter(mm, MM_SWAPENTS);
>  			swp_pte = swp_entry_to_pte(entry);