Re: [RFC PATCH V2 03/13] mm: Scan the mm and create a migration list

[Harry Yoo]

On Thu, Jun 26, 2025 at 07:07:12AM +0900, Harry Yoo wrote:
> On Tue, Jun 24, 2025 at 05:56:07AM +0000, Raghavendra K T wrote:
> > Since we already have the list of mm_struct in the system, add a module to
> > scan each mm that walks VMAs of each mm_struct and scan all the pages
> > associated with that.
> > 
> >  In the scan path: Check for the recently acccessed pages (folios) belonging
> > to slowtier nodes. Add all those folios to a list.
> > 
> > Signed-off-by: Raghavendra K T <raghavendra.kt@xxxxxxx>
> > ---
> 
> Hi, just taking a quick look...
> 
> >  mm/kscand.c | 319 +++++++++++++++++++++++++++++++++++++++++++++++++++-
> >  1 file changed, 318 insertions(+), 1 deletion(-)
> > 
> > diff --git a/mm/kscand.c b/mm/kscand.c
> > index d5b0d3041b0f..0edec1b7730d 100644
> > --- a/mm/kscand.c
> > +++ b/mm/kscand.c
> > @@ -42,6 +55,8 @@ static struct kmem_cache *kscand_slot_cache __read_mostly;
> > @@ -84,11 +122,275 @@ static void kscand_wait_work(void)
> >  			scan_sleep_jiffies);
> >  }
> >  
> > +static inline bool is_valid_folio(struct folio *folio)
> > +{
> > +	if (!folio || folio_test_unevictable(folio) || !folio_mapped(folio) ||
> > +		folio_is_zone_device(folio) || folio_maybe_mapped_shared(folio))
> > +		return false;
> > +
> > +	return true;
> > +}
> 
> What makes it undesirable to migrate shared folios?
> 
> > +static bool folio_idle_clear_pte_refs_one(struct folio *folio,
> > +					 struct vm_area_struct *vma,
> > +					 unsigned long addr,
> > +					 pte_t *ptep)
> > +{
> > +	bool referenced = false;
> > +	struct mm_struct *mm = vma->vm_mm;
> > +	pmd_t *pmd = pmd_off(mm, addr);
> > +
> > +	if (ptep) {
> > +		if (ptep_clear_young_notify(vma, addr, ptep))
> > +			referenced = true;
> > +	} else if (IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE)) {
> > +		if (!pmd_present(*pmd))
> > +			WARN_ON_ONCE(1);
> > +		if (pmdp_clear_young_notify(vma, addr, pmd))
> > +			referenced = true;
> > +	} else {
> > +		WARN_ON_ONCE(1);
> > +	}
> 
> This does not look good.
> 
> I think pmd entry handling should be handled in
> mm_walk_ops.pmd_entry callback?
> 
> > +
> > +	if (referenced) {
> > +		folio_clear_idle(folio);
> > +		folio_set_young(folio);
> > +	}
> > +
> > +	return true;
> > +}
> > +
> > +static void page_idle_clear_pte_refs(struct page *page, pte_t *pte, struct mm_walk *walk)
> > +{
> > +	bool need_lock;
> > +	struct folio *folio =  page_folio(page);
> > +	unsigned long address;
> > +
> > +	if (!folio_mapped(folio) || !folio_raw_mapping(folio))
> > +		return;
> > +
> > +	need_lock = !folio_test_anon(folio) || folio_test_ksm(folio);
> > +	if (need_lock && !folio_trylock(folio))
> > +		return;
> 
> Why acquire folio lock here?
>
> And I'm not even sure if it's safe to acquire it?
> The locking order is folio_lock -> pte_lock
>
> page walk should have already acquired pte_lock before calling
> ->pte_entry() callback.

Oops, it's trylock. Nevermind.
Needed more coffee in the morning :)
 
> > +	address = vma_address(walk->vma, page_pgoff(folio, page), compound_nr(page));
> > +	VM_BUG_ON_VMA(address == -EFAULT, walk->vma);
> > +	folio_idle_clear_pte_refs_one(folio, walk->vma, address, pte);
> > +
> > +	if (need_lock)
> > +		folio_unlock(folio);
> > +}
> > +
> > +static const struct mm_walk_ops hot_vma_set_idle_ops = {
> > +	.pte_entry = hot_vma_idle_pte_entry,
> > +	.walk_lock = PGWALK_RDLOCK,
> > +};
> > +
> > +static void kscand_walk_page_vma(struct vm_area_struct *vma, struct kscand_scanctrl *scanctrl)
> > +{
> > +	if (!vma_migratable(vma) || !vma_policy_mof(vma) ||
> > +	    is_vm_hugetlb_page(vma) || (vma->vm_flags & VM_MIXEDMAP)) {
> > +		return;
> > +	}
> > +	if (!vma->vm_mm ||
> > +	    (vma->vm_file && (vma->vm_flags & (VM_READ|VM_WRITE)) == (VM_READ)))
> > +		return;
> 
> Why not walk writable file VMAs?
> 
> > +	if (!vma_is_accessible(vma))
> > +		return;
> > +
> > +	walk_page_vma(vma, &hot_vma_set_idle_ops, scanctrl);
> > +}
> 
> -- 
> Cheers,
> Harry / Hyeonggon

-- 
Cheers,
Harry / Hyeonggon