Re: [PATCH v2] mm/migrate: improve mlock tracking for PTE-mapped large folios

[Kiryl Shutsemau <kas@kernel.org>]

On Wed, Nov 11, 2026 at 09:00:10AM +0100, David Hildenbrand (Arm) wrote:
> On 11/10/26 13:32, Wandun Chen wrote:
> > From: Wandun Chen <chenwandun@lixiang.com>
> > 
> > Commit ab521b4142aa ("mm/rmap: improve mlock tracking for large folios")
> > mlocks a large folio when all its pages are added in one rmap operation.
> > However, when a PTE-mapped large folio is migrated, remove_migration_pte()
> > restores PTEs individually, passing nr_pages = 1 to the rmap helpers. As a
> > result, PG_mlocked is not set on the destination folio.
> > 
> > Reclaim can restore mlock in try_to_unmap(), but Mlocked in /proc/meminfo
> > remains undercounted until that scan occurs.
> > 
> > Restore mlock once the full mapping has been restored in a locked VMA
> > under one page table lock, rather than waiting for try_to_unmap() to
> > restore the mlock state.
> > 
> > Signed-off-by: Wandun Chen <chenwandun@lixiang.com>
> > ---
> >  mm/migrate.c | 10 +++++++++-
> >  1 file changed, 9 insertions(+), 1 deletion(-)
> > 
> > diff --git a/mm/migrate.c b/mm/migrate.c
> > index 94033c222e04..140e291d4587 100644
> > --- a/mm/migrate.c
> > +++ b/mm/migrate.c
> > @@ -353,6 +353,7 @@ static bool remove_migration_pte(struct folio *folio,
> >  {
> >  	struct rmap_walk_arg *rmap_walk_arg = arg;
> >  	DEFINE_FOLIO_VMA_WALK(pvmw, rmap_walk_arg->folio, vma, addr, PVMW_SYNC | PVMW_MIGRATION);
> > +	unsigned long nr_ptes = 0;
> >  
> >  	while (page_vma_mapped_walk(&pvmw)) {
> >  		rmap_t rmap_flags = RMAP_NONE;
> > @@ -446,9 +447,16 @@ static bool remove_migration_pte(struct folio *folio,
> >  			else
> >  				folio_add_file_rmap_pte(folio, new, vma);
> >  			set_pte_at(vma->vm_mm, pvmw.address, pvmw.pte, pte);
> > +
> > +			nr_ptes++;
> >  		}
> > -		if (READ_ONCE(vma->vm_flags) & VM_LOCKED)
> > +		if (READ_ONCE(vma->vm_flags) & VM_LOCKED) {
> > +			if (folio_test_large(folio) &&
> > +			    nr_ptes == folio_nr_pages(folio) &&
> > +			    !(pvmw.flags & PVMW_PGTABLE_CROSSED))
> > +				mlock_vma_folio(folio, vma);
> >  			mlock_drain_local();
> > +		}
> >  
> >  		trace_remove_migration_pte(pvmw.address, pte_val(pte),
> >  					   compound_order(new));
> 
> I guess pte batching would be one way of handling it, but that requires more churn.
> 
> The spirit is similar as in try_to_unmap_one(), where we count the number of
> ptes in one VMA. For consistency, I would call it "ptes".
> 
> @Kiryl, can you take a look?

Looks right to me. Agreed on "ptes".

With that adjusted:

Reviewed-by: Kiryl Shutsemau (Meta) <kas@kernel.org>

> 
> BTW, why don't we care about that in try_to_migrate?

try_to_migrate_one() unmaps the folio whether the VMA is VM_LOCKED or
not, so there is no mapping left to keep mlocked; removing the rmap
munlocks the source.

The mlock is rebuilt on the destination in remove_migration_ptes(),
which is the path this patch fixes.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov