Re: [PATCH v1 2/2] mm/memory: add anonymous mTHP folios to deferred split list

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

On 7/7/26 22:17, Joanne Koong wrote:
> Unlike for PMD-sized folios, an anonymous mTHP folio doesn't get added
> to the deferred split list at fault or collapse time. As a result, a
> fully mapped mTHP folio that is mostly zero-filled doesn't get split by
> the deferred split shrinker when the system is under memory pressure.
> 
> Add anonymous mTHP folios to the deferred split list so that if there's
> memory pressure, a zero-filled mTHP can be split with its zero pages
> remapped to the shared zero page and then reclaimed.
> 
> To minimize overhead on the common order-0 fault path, the
> deferred_split_folio() call is guarded by an inline folio_test_large()
> check.
> 
> Suggested-by: Usama Arif <usama.arif@linux.dev>
> Signed-off-by: Joanne Koong <joannelkoong@gmail.com>
> ---
>  mm/memory.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/mm/memory.c b/mm/memory.c
> index 6637c5b13c9b..441d918e3dc0 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -5259,6 +5259,8 @@ void map_anon_folio_pte_nopf(struct folio *folio, pte_t *pte,
>  	folio_add_lru_vma(folio, vma);
>  	set_ptes(vma->vm_mm, addr, pte, entry, nr_pages);
>  	update_mmu_cache_range(NULL, vma, addr, pte, nr_pages);
> +	if (folio_test_large(folio))
> +		deferred_split_folio(folio, false);
>  }
>  
>  static void map_anon_folio_pte_pf(struct folio *folio, pte_t *pte,

I had a session [1] at LSF/MM about having essentially all large anon folios
part of the the deferred split queue.

(1) I don't think this scales.

(2) I suspect the shrinker should make smarter decisions of what to scan/reclaim
first.

I think this needs more proper thought.

[1]
https://docs.google.com/presentation/d/1RfKWCY1AMVns-WLn-QdAWbI2a-rA7fbFyh7XD1Wn5BY/edit?usp=sharing

-- 
Cheers,

David