Re: [RFC] mm: restrict zero-page remapping to underused THP splits

["David Hildenbrand (Arm)" <david@kernel.org>]

On 5/11/26 20:40, Nico Pache wrote:
> On Fri, May 8, 2026 at 3:32 PM David Hildenbrand (Arm) <david@kernel.org> wrote:
>>
>> On 5/8/26 19:05, Nico Pache wrote:
>>> Since commit b1f202060afe ("mm: remap unused subpages to shared zeropage
>>> when splitting isolated thp"), splitting an anonymous THP remaps all
>>> zero-filled subpages to the shared zeropage via TTU_USE_SHARED_ZEROPAGE.
>>> This flag is set unconditionally for every anonymous folio split,
>>> including splits triggered by KSM.
>>
>> And even when the underused scanner is effectively disabled on a system. Hm.
>>
>> I don't quite like that we scan for zeropages when nobody even requested us to
>> split because of zeropages.
>>
>> I can see why we would want to scan for zeropages in a setup where the underused
>> scanner is active, even when the split was triggered by someone/something else
>> (below).
>>
>> [...]
>>
>>>  /**
>>> @@ -4340,7 +4341,13 @@ int folio_split(struct folio *folio, unsigned int new_order,
>>>               struct page *split_at, struct list_head *list)
>>>  {
>>>       return __folio_split(folio, new_order, split_at, &folio->page, list,
>>> -                          SPLIT_TYPE_NON_UNIFORM);
>>> +                          SPLIT_TYPE_NON_UNIFORM, false);
>>> +}
>>> +
>>> +int folio_split_underused(struct folio *folio)
>>> +{
>>> +     return __folio_split(folio, 0, &folio->page, &folio->page,
>>> +                          NULL, SPLIT_TYPE_NON_UNIFORM, true);
>>>  }
>>>
>>>  /**
>>> @@ -4559,7 +4566,7 @@ static unsigned long deferred_split_scan(struct shrinker *shrink,
>>>               }
>>>               if (!folio_trylock(folio))
>>>                       goto requeue;
>>> -             if (!split_folio(folio)) {
>>> +             if (!folio_split_underused(folio)) {
>>>                       did_split = true;
>>>                       if (underused)
>>>                               count_vm_event(THP_UNDERUSED_SPLIT_PAGE);
>>
>> In general, this looks clean.
>>
>> But imagine the following: someone splits the THP for another reason: for
>> example, because migration is unable to allocate a 2M THP, or because we have to
>> split on swapout etc.
>>
>> Not freeing the zero-filled pages means that these pages cannot be reclaimed
>> anymore easily. We split a possibly underused THP but didn't free the memory.
>>
>> The only way to free the memory would be to wait for another collapse, and then
>> have the new THP be detected as underused.
>>
>> Hm.
> 
> And what was the expected behavior before this commit? Did we just
> deal with the wasted memory?

Before your change, splitting will always free memory, no matter who triggers
splitting. So there is no wasted memory (regarding underused THPs).

With your change, if we happen to split before the deferred shrinker runs, we
end up with zero-filled pages that waste memory. And reclaiming them (through
the deferred shrinker) first requires another re-collapse to a THP.

Or am I misunderstanding your question?

> 
>>
>> (1) As you say, the alternative is to let KSM say that it wants to handle the
>> zero-filled pages itself. I'm not a the biggest fan of that approach. We still
>> have two mechanisms interacting to some degree.
>>
>> (2) Another approach is to just let KSM handle this in VMAs that are marked as
>> mergable while KSM is active. That is, we check for VM_MERGABLE and ksm_run ==
>> KSM_RUN_MERGE in try_to_map_unused_to_zeropage() to just let KSM do its thing.
>>
>> That really just stops both mechanisms from interacting.
>>
>> (3) Yet another approach I could think of (in general) is to disable the
>> underused handling in a system where the underused splitting is entirely disabled.
>>
>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
>> index e9d499da0ac7..5eca99271957 100644
>> --- a/mm/huge_memory.c
>> +++ b/mm/huge_memory.c
>> @@ -82,6 +82,14 @@ unsigned long huge_anon_orders_madvise __read_mostly;
>>  unsigned long huge_anon_orders_inherit __read_mostly;
>>  static bool anon_orders_configured __initdata;
>>
>> +static bool thp_underused_split_active(void)
>> +{
>> +       if (!split_underused_thp)
>> +               return false;
>> +
>> +       return khugepaged_max_ptes_none != HPAGE_PMD_NR - 1;
>> +}
>> +
>>  static inline bool file_thp_enabled(struct vm_area_struct *vma)
>>  {
>>         struct inode *inode;
>> @@ -4188,7 +4196,8 @@ static int __folio_split(struct folio *folio, unsigned int
>> new_order,
>>         if (nr_shmem_dropped)
>>                 shmem_uncharge(mapping->host, nr_shmem_dropped);
>>
>> -       if (!ret && is_anon && !folio_is_device_private(folio))
>> +       if (!ret && is_anon && !folio_is_device_private(folio) &&
>> +           thp_underused_split_active())
>>                 ttu_flags = TTU_USE_SHARED_ZEROPAGE;
>>
>>         remap_page(folio, 1 << old_order, ttu_flags);
>> @@ -4497,7 +4506,7 @@ static bool thp_underused(struct folio *folio)
>>         int num_zero_pages = 0, num_filled_pages = 0;
>>         int i;
>>
>> -       if (khugepaged_max_ptes_none == HPAGE_PMD_NR - 1)
>> +       if (!thp_underused_split_active())
>>                 return false;
>>
>>         if (folio_contain_hwpoisoned_page(folio))
>>
>>
>>
>> I tend to like (2), and maybe (3) on top. Opinions?
> 
> I don't fully understand (2) but I definitely agree with (3).
> 
> Isn't (2) similar to my split_huge_page_no_zeropage() solution in that
> it only disables the behavior for KSM? but instead handled much
> further down in the call path? The "fix" commit sold this as a
> solution ONLY for the underutilized shrinker, but it is not that.

It keeps other paths that happen to split the folio before the deferred shrinker
runs unaffected (see above).

-- 
Cheers,

David