Re: [PATCH v3 6/6] mm: khugepaged: set to next mm direct when mm has MMF_DISABLE_THP_COMPLETELY

[Lance Yang]

On 2026/1/5 08:31, Wei Yang wrote:
> On Sun, Jan 04, 2026 at 08:20:29PM +0800, Lance Yang wrote:
> > On 2026/1/4 13:41, Vernon Yang wrote:
> > > When an mm with the MMF_DISABLE_THP_COMPLETELY flag is detected during
> > > scanning, directly set khugepaged_scan.mm_slot to the next mm_slot,
> > > reduce redundant operation.
> > >
> > > Signed-off-by: Vernon Yang <yanglincheng@xxxxxxxxxx>
> > > ---
> > >    mm/khugepaged.c | 2 +-
> > >    1 file changed, 1 insertion(+), 1 deletion(-)
> > >
> > > diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> > > index 1ca034a5f653..d4ed0f397335 100644
> > > --- a/mm/khugepaged.c
> > > +++ b/mm/khugepaged.c
> > > @@ -2541,7 +2541,7 @@ static unsigned int khugepaged_scan_mm_slot(unsigned int pages, int *result,
> > >    	 * Release the current mm_slot if this mm is about to die, or
> > >    	 * if we scanned all vmas of this mm.
> > >    	 */
> > > -	if (hpage_collapse_test_exit(mm) || !vma) {
> > > +	if (hpage_collapse_test_exit_or_disable(mm) || !vma) {
> > >    		/*
> > >    		 * Make sure that if mm_users is reaching zero while
> > >    		 * khugepaged runs here, khugepaged_exit will find
> >
> >
> > Let's convert hpage_collapse_test_exit() in collect_mm_slot() as well,
> > otherwise the mm_slot would not be freed and will be scanned again ...
> >
> > static void collect_mm_slot(struct mm_slot *slot)
> > {
> > 	struct mm_struct *mm = slot->mm;
> >
> > 	lockdep_assert_held(&khugepaged_mm_lock);
> >
> > 	if (hpage_collapse_test_exit(mm)) { <-
>
> What if user toggle the MMF_DISABLE_THP_COMPLETELY flag again?

Maybe it's fine :)

If user sets MMF_DISABLE_THP_COMPLETELY, they probaly would not
clear it soon. Keeping the slot wastes memory.

If they do clear it later, page faults will trigger
do_huge_pmd_anonymous_page() -> khugepaged_enter_vma(), which
re-adds the mm.

Anyway, no strong opinion on that.

> > 		hash_del(&slot->hash);
> > 		list_del(&slot->mm_node);
> >
> > 		mm_slot_free(mm_slot_cache, slot);
> > 		mmdrop(mm);
> > 	}
> > }