Re: [PATCH mm-hotfixes-unstable v18 00/14] khugepaged: add mTHP collapse support

[Nico Pache]

On Mon, Jun 1, 2026 at 9:58 AM Alexander Gordeev <agordeev@xxxxxxxxxxxxx> wrote:
>
> On Fri, May 22, 2026 at 01:47:24PM -0700, Andrew Morton wrote:
>
> Hi Andrew et al,
>
> > On Fri, 22 May 2026 08:59:55 -0600 Nico Pache <npache@xxxxxxxxxx> wrote:
> >
> > > The following series provides khugepaged with the capability to collapse
> > > anonymous memory regions to mTHPs.
> >
> > Thanks, I've update mm.git's mm-unstable branch to this version.
> >
> > It sounds like I might be dropping it soon, haven't started looking at
> > that yet.  But let's at least eyeball the latest version at this time.
> >
> > Sashiko was able to apply this, so the base-it-on-hotfixes thing worked
> > well, thanks.  The AI checking made a few allegations:
>
> This series appears to cause hangs on s390 in linux-next.
> The issue is not easily reproducible, so it is not yet confirmed.
> Any ideas for a reliable reproducer that exercises the code path below?

Hi,

Thanks for the report!

was this caught by syzbot? If so, can you provide a link?

Also can you provide whether any of the mTHP sysfs settings were enabled?

Based on the report, it looks like we are either dealing with more
lock contention (due to holding the write lock longer). We could
switch to a trylock but that might cause us to lose some collapse
attempts (which will be retried later, so probably fine). I'm ok with
that approach if it prevents these potential regressions.

Cheers,
-- Nico

>
>     [ 2749.385719] sysrq: Show Blocked State
>     [ 2749.385730] task:khugepaged      state:D stack:0     pid:209   tgid:209   ppid:2      task_flags:0x200040 flags:0x00000000
>     [ 2749.385735] Call Trace:
>     [ 2749.385736]  [<0000017f63c8b226>] __schedule+0x316/0x890
>     [ 2749.385740]  [<0000017f63c8b7dc>] schedule+0x3c/0xc0
>     [ 2749.385743]  [<0000017f63c8b888>] schedule_preempt_disabled+0x28/0x40
>     [ 2749.385746]  [<0000017f63c902ea>] rwsem_down_write_slowpath+0x2fa/0x8b0
>     [ 2749.385749]  [<0000017f63c90910>] down_write+0x70/0x80
>     [ 2749.385752]  [<0000017f6313407a>] collapse_huge_page+0x2ea/0x9e0
>     [ 2749.385755]  [<0000017f6313491e>] mthp_collapse+0x1ae/0x1f0
>     [ 2749.385757]  [<0000017f63134fda>] collapse_scan_pmd+0x67a/0x8f0
>     [ 2749.385760]  [<0000017f6313751a>] collapse_single_pmd+0x15a/0x260
>     [ 2749.385762]  [<0000017f6313792c>] collapse_scan_mm_slot.constprop.0+0x30c/0x470
>     [ 2749.385765]  [<0000017f63137cb6>] khugepaged+0x226/0x240
>     [ 2749.385768]  [<0000017f62db3128>] kthread+0x148/0x170
>     [ 2749.385770]  [<0000017f62d2c238>] __ret_from_fork+0x48/0x220
>     [ 2749.385772]  [<0000017f63c95d0a>] ret_from_fork+0xa/0x30
>
> Thanks!
>