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

[David Hildenbrand (Red Hat)]

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?

--
Cheers

David