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

[David Hildenbrand (Arm)]

On 5/13/26 19:21, Usama Arif wrote:
> 
> 
> 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@xxxxxxxxx>
>>> ---
>>>  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!

Planning on doing more ...

> 
> 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.

Maybe the clearest thing is mm_prepare_for_swap_entries(), because that's what
we're about to establish in these code paths: swap entries.

I wouldn't mention swapoff, but both are better than ensure_on_mmlist ;)

-- 
Cheers,

David