Re: [PATCH 2/4] mm: khugepaged: remove mm when all memory has been collapsed

[Vernon Yang <vernon2gm@gmail.com>]

On Mon, Dec 15, 2025 at 07:52:41PM +0800, Lance Yang wrote:
> Hi Vernon,
>
> Thanks for the patches!
>
> On 2025/12/15 17:04, Vernon Yang wrote:
> > The following data is traced by bpftrace on a desktop system. After
> > the system has been left idle for 10 minutes upon booting, a lot of
> > SCAN_PMD_MAPPED or SCAN_PMD_NONE are observed during a full scan by
> > khugepaged.
> >
> > @scan_pmd_status[1]: 1           ## SCAN_SUCCEED
> > @scan_pmd_status[4]: 158         ## SCAN_PMD_MAPPED
> > @scan_pmd_status[3]: 174         ## SCAN_PMD_NONE
> > total progress size: 701 MB
> > Total time         : 440 seconds ## include khugepaged_scan_sleep_millisecs
> >
> > The khugepaged_scan list save all task that support collapse into hugepage,
> > as long as the take is not destroyed, khugepaged will not remove it from
>
> Nit: s/take/task/

Thanks, I'll fix it in the next version.

> > the khugepaged_scan list. This exist a phenomenon where task has already
> > collapsed all memory regions into hugepage, but khugepaged continues to
> > scan it, which wastes CPU time and invalid, and due to
> > khugepaged_scan_sleep_millisecs (default 10s) causes a long wait for
> > scanning a large number of invalid task, so scanning really valid task
> > is later.
> >
> > After applying this patch, when all memory is either SCAN_PMD_MAPPED or
> > SCAN_PMD_NONE, the mm is automatically removed from khugepaged's scan
> > list. If the page fault or MADV_HUGEPAGE again, it is added back to
> > khugepaged.
> >
> > Signed-off-by: Vernon Yang <yanglincheng@kylinos.cn>
> > ---
> >   mm/khugepaged.c | 35 +++++++++++++++++++++++++----------
> >   1 file changed, 25 insertions(+), 10 deletions(-)
> >
> > diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> > index 0598a19a98cc..1ec1af5be3c8 100644
> > --- a/mm/khugepaged.c
> > +++ b/mm/khugepaged.c
> > @@ -115,6 +115,7 @@ struct khugepaged_scan {
> >   	struct list_head mm_head;
> >   	struct mm_slot *mm_slot;
> >   	unsigned long address;
> > +	bool maybe_collapse;
>
> At a quick glance, the name of "maybe_collapse" is a bit ambiguous ...
>
> Perhaps "scan_needed" or "collapse_possible" would be clearer to
> indicate that the mm should be kept in the scan list?

The "collapse_possible" sounds good to me, Thanks! I will do it in the
next version.

--
Thanks,
Vernon