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

[Usama Arif <usama.arif@linux.dev>]

On 13/05/2026 14:32, David Hildenbrand (Arm) wrote:
> On 4/27/26 12:01, 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@linux.dev>
>> ---
>>  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);
>> +	}
>> +}
> 
> Instead of talking about the low level detail ("add to mmlist"), maybe we could
> just talk about the high-level goal: make sure that the MM can hold swap entries.
> 
> 
> mm_prepare_for_swap()
> 
> or sth like that?
> 

Thanks for the review!

Ah so basically rename the function to mm_prepare_for_swap(). I felt like it
makes the function sound more important than it is? But it is a better
name than ensure_on_mmlist(). Maybe mm_prepare_for_swapoff()? As the mmlist
is only used for swapoff.