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

[Minchan Kim <minchan@kernel.org>]

On Tue, Apr 14, 2026 at 09:20:25AM +0200, David Hildenbrand (Arm) wrote:
> 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@kernel.org>
> > ---
> >  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.

Didn't know that. I will use folio_maybe_mapped_shared in next revision.

Thank you!