On 24 Sep 2026, at 6:21, Lorenzo Stoakes (ARM) wrote:
> 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.
Your reasoning makes sense to me.
>
>>
>>> 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.
Sure. It is great learning material for me. Thank you for the patches.
>
> 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!
Of course, great to see hacky code being removed by this series.
For this patch, feel free to add
Reviewed-by: Zi Yan <[email protected]>
Best Regards,
Yan, Zi
