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

["David Hildenbrand (Red Hat)" <david@kernel.org>]

On 12/19/25 09:35, 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?

I'd have to see the full patch, but IMHO we should rather focus on on 
"how many pte/pmd entries did we check" and not "how many PMD areas did 
we check".

Maybe there is a history to this, but conceptually I think we wanted to 
limit the work we do in one operation to something reasonable. Reading a 
single PMD is obviously faster than 512 PTEs.

-- 
Cheers

David