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

[Vernon Yang <vernon2gm@gmail.com>]

On Thu, Dec 18, 2025 at 10:29:18AM +0100, David Hildenbrand (Red Hat) wrote:
> On 12/15/25 10:04, 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
> > 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.
>
> I don't like that, as it assumes that memory within such a process would be
> rather static, which is easily not the case (e.g., allocators just doing
> MADV_DONTNEED to free memory).
>
> If most stuff is collapsed to PMDs already, can't we just skip over these
> regions a bit faster?

I have a flash of inspiration and came up with a good idea.

If these regions have already been collapsed into hugepage, rechecking
them would be very fast. Due to the khugepaged_pages_to_scan can also
represent the number of VMAs to skip, we can extend its semantics as
follows:

	/*
	 * default scan 8*HPAGE_PMD_NR ptes, pmd_mapped, no_pte_table or vmas
	 * every 10 second.
	 */
	static unsigned int khugepaged_pages_to_scan __read_mostly;

	switch (*result) {
	case SCAN_NO_PTE_TABLE:
	case SCAN_PMD_MAPPED:
	case SCAN_PTE_MAPPED_HUGEPAGE:
		progress++; // here
		break;
	case SCAN_SUCCEED:
		++khugepaged_pages_collapsed;
		fallthrough;
	default:
		progress += HPAGE_PMD_NR;
	}

This way can achieve our goal. David, do you like it?

> --
> Cheers
>
> David

--
Thanks,
Vernon