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

[Vernon Yang <vernon2gm@gmail.com>]

On Tue, Dec 23, 2025 at 04:48:57PM +0530, Dev Jain wrote:
>
> On 19/12/25 2:05 pm, Vernon Yang wrote:
> > 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?
>
> This looks good, can you formally test this and see if it comes close to the optimizations
> yielded by the current version of the patchset?

Both can achieve this function, reducing the time of a full scan,
previously tested.

About performance test, I will test it formally.

--
Merry Christmas,
Vernon