Re: [RFC 2/3] mm: process_mrelease: skip LRU movement for exclusive file folios

[David Hildenbrand (Arm)]

On 4/14/26 00:39, Minchan Kim wrote:
> For the process_mrelease reclaim, skip LRU handling for exclusive
> file-backed folios since they will be freed soon so pointless
> to move around in the LRU.
> 
> This avoids costly LRU movement which accounts for a significant portion
> of the time during unmap_page_range.
> 
> -   91.31%     0.00%  mmap_exit_test   [kernel.kallsyms]  [.] exit_mm
>      exit_mm
>      __mmput
>      exit_mmap
>      unmap_vmas
>    - unmap_page_range
>       - 55.75% folio_mark_accessed
>          + 48.79% __folio_batch_add_and_move
>            4.23% workingset_activation
>       + 12.94% folio_remove_rmap_ptes
>       + 9.86% page_table_check_clear
>       + 3.34% tlb_flush_mmu
>         1.06% __page_table_check_pte_clear
> 
> Signed-off-by: Minchan Kim <minchan@xxxxxxxxxx>
> ---
>  mm/memory.c | 13 ++++++++++++-
>  1 file changed, 12 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/memory.c b/mm/memory.c
> index 2f815a34d924..25e17893c919 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -1640,6 +1640,8 @@ static __always_inline void zap_present_folio_ptes(struct mmu_gather *tlb,
>  	bool delay_rmap = false;
>  
>  	if (!folio_test_anon(folio)) {
> +		bool skip_mark_accessed;
> +
>  		ptent = get_and_clear_full_ptes(mm, addr, pte, nr, tlb->fullmm);
>  		if (pte_dirty(ptent)) {
>  			folio_mark_dirty(folio);
> @@ -1648,7 +1650,16 @@ static __always_inline void zap_present_folio_ptes(struct mmu_gather *tlb,
>  				*force_flush = true;
>  			}
>  		}
> -		if (pte_young(ptent) && likely(vma_has_recency(vma)))
> +
> +		/*
> +		 * For the process_mrelease reclaim, skip LRU handling for exclusive
> +		 * file-backed folios since they will be freed soon so pointless
> +		 * to move around in the LRU.
> +		 */
> +		skip_mark_accessed = mm_flags_test(MMF_UNSTABLE, mm) &&
> +				     folio_mapcount(folio) < 2;

folio_mapcount() is most certainly the wrong thing to use if you want to
handle large folios properly.

Maybe !folio_likely_mapped_shared() is what you are looking for. Maybe.

-- 
Cheers,

David