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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <uqlhxbrsl36quvxaflrueadujb3cwdfexw4yffvk6la3ry4szj@h62lqtp36g5c>
Date: Tue, 16 Dec 2025 14:27:05 +0800
From: Vernon Yang <vernon2gm@...il.com>
To: Lance Yang <lance.yang@...ux.dev>
Cc: ziy@...dia.com, baohua@...nel.org, linux-mm@...ck.org, 
	linux-kernel@...r.kernel.org, Vernon Yang <yanglincheng@...inos.cn>, akpm@...ux-foundation.org, 
	lorenzo.stoakes@...cle.com, david@...nel.org
Subject: Re: [PATCH 2/4] mm: khugepaged: remove mm when all memory has been
 collapsed

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@...inos.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