On Wed, Sep 23, 2026 at 04:06:14PM -0400, Zi Yan wrote:
> On 17 Sep 2026, at 12:22, Lorenzo Stoakes (ARM) wrote:
>
> > When performing mlock() or munlock() otherwise normal VMAs have VMA_IO_BIT
> > solely to fix a race with migration which might otherwise double-count
> > mlock VMAs.
> >
> > This is unnecessary - at the point of applying folio mlock state, whether
> > setting or clearing PG_mlocked, we know whether or not we are locking.
> >
> > Solve this in two ways - thread a boolean through the page table walk
> > indicating whether a lock or unlock is being performed, and run a locking
> > walk with VMA_LOCKONFAULT_BIT set and VMA_LOCKED_BIT cleared.
> >
> > This state never occurs otherwise, as VMA_LOCKONFAULT_BIT always implies
> > VMA_LOCKED_BIT. These are also always cleared together.
> >
> > Then, update folio_add_lru_vma() and mlock_folio() to check only for
> > VMA_LOCKED_BIT, and update try_to_unmap_one() to check for VMA_LOCKED_MASK
> > instead.
> >
> > Also remove the useless invocation of allow_mlock_munlock() which simply
> > returns true if unlocking and instead rename it to allow_mlock() and only
> > call it when locking.
> >
> > Finally, with the other mlock abuse of VMA_IO_BIT addressed, update
> > mlock_vma_folio() and folio_add_lru_vma() to simply test for
> > VMA_LOCKED_BIT. munlock_vma_folio() tests VMA_LOCKED_MASK instead, as an
> > unmap racing with the locking walk must still munlock folios the walk has
> > already counted.
> >
> > While here, also replace some deprecated VMA flag predicates.
> >
> > Signed-off-by: Lorenzo Stoakes (ARM) <[email protected]>
> > ---
> > mm/folio.c | 2 +-
> > mm/internal.h | 10 +++++++---
> > mm/mlock.c | 51 +++++++++++++++++++--------------------------------
> > mm/rmap.c | 4 +++-
> > 4 files changed, 30 insertions(+), 37 deletions(-)
> >
> > diff --git a/mm/folio.c b/mm/folio.c
> > index 47a437e0f7fd..35e242b48870 100644
> > --- a/mm/folio.c
> > +++ b/mm/folio.c
> > @@ -505,7 +505,7 @@ void folio_add_lru_vma(struct folio *folio, struct
> > vm_area_struct *vma)
> > {
> > VM_BUG_ON_FOLIO(folio_test_lru(folio), folio);
> >
> > - if (unlikely((vma->vm_flags & (VM_LOCKED | VM_SPECIAL)) == VM_LOCKED))
> > + if (vma_test(vma, VMA_LOCKED_BIT))
>
> I think it is worth documenting VMA_LOCKONFAULT_BIT alone means mlock in
> progress, like you did in munlock_vma_folio(). Just to keep the protocol
> explicit for all the readers.
Well I'm not sure it's necessary here honestly, because this never checked
VMA_LOCKED_MASK anyway, and VMA_LOCKONFAULT_BIT never made a difference.
So the meaning of VMA_LOCKED_BIT here is strictly 'is it locked' and it's
correctly handled.
And I fear that it becomes whack-a-mole - the neat thing about this change is
that you no longer have to special case the stupid VM_SPECIAL thing, and can in
fact do the 'normal' thing of _just checking_ VMA_LOCKED_BIT :)
So I think it's better not to.
>
> > mlock_new_folio(folio);
> > else
> > folio_add_lru(folio);
> > diff --git a/mm/internal.h b/mm/internal.h
> > index b2c6c9435021..84aa3e6c8bac 100644
> > --- a/mm/internal.h
> > +++ b/mm/internal.h
> > @@ -971,8 +971,7 @@ void mlock_folio(struct folio *folio);
> > static inline void mlock_vma_folio(struct folio *folio,
> > struct vm_area_struct *vma)
> > {
> > - /* The VM_IO check prevents migration from double-counting during
> > mlock. */
> > - if (unlikely((vma->vm_flags & (VM_LOCKED|VM_SPECIAL)) == VM_LOCKED))
> > + if (vma_test(vma, VMA_LOCKED_BIT))
>
> Ditto.
Similar reasoning to above.
>
> > mlock_folio(folio);
> > }
> >
> > @@ -989,7 +988,12 @@ static inline void munlock_vma_folio(struct folio
> > *folio,
> > * always munlock the folio and page reclaim will correct it
> > * if it's wrong.
> > */
> > - if (unlikely(vma->vm_flags & VM_LOCKED))
> > + /*
> > + * VMA_LOCKONFAULT_BIT alone marks an mlock walk in progress, see
> > + * mlock_vma_pages_range(). An unmap racing with the walk must still
> > + * munlock folios the walk has already counted.
> > + */
Here it's worth mentioning, as it's specifically relying on the new
behaviour. Although it's neatly using the VMA_LOCKED_MASK to handle both the
locked case and the 'being locked' case :)
> > + if (unlikely(vma_test_any_mask(vma, VMA_LOCKED_MASK)))
> > munlock_folio(folio);
> > }
> >
>
> Why I am commenting in the middle of the series? Because I am taking
> a quiz given by LLM based on this series to get myself enough background
> knowledge to review this series. This mlock part came up at part E
> and I only have part F left before I can do the full review. :)
Thanks! :) I really appreciate you taking the time to look at this! Sorry it's
so large.
I held this series back from last cycle to help with review load, then spent
some time fixing various AI-discovered things, and all the patches are necessary
(well for the most part) to get where the series needs to go.
I think the change is worth it though!
>
> Best Regards,
> Yan, Zi
--
Cheers, Lorenzo
