Re: [PATCHv2] mm: khugepaged: make scan loops suspend aware

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

On 2/11/26 04:15, Sergey Senozhatsky wrote:
> A number of khugepaaged's loops, e.g. khugepaged_scan_mm_slot(),
> are time unbound, which can become problematic during system
> suspend:
> 
> PM: suspend entry (s2idle)
> Filesystems sync: 0.003 seconds
> Freezing user space processes
> Freezing user space processes completed (elapsed 0.003 seconds)
> OOM killer disabled.
> Freezing remaining freezable tasks
> Freezing remaining freezable tasks failed after 20.004 seconds (1 tasks refusing to freeze, wq_busy=0):
> task:khugepaged      state:D stack:0     pid:1345  ppid:2      flags:0x00004000
> Call Trace:
>   <TASK>
>   schedule+0x523/0x16a0
>   schedule_timeout+0x23b/0x6e0
>   io_schedule_timeout+0x3f/0x80
>   wait_for_completion_io_timeout+0xe4/0x170
>   submit_bio_wait+0x79/0xc0
>   swap_readpage+0x150/0x2d0
>   swap_cluster_readahead+0x3be/0x750
>   shmem_swapin+0xa7/0x100
>   shmem_swapin_folio+0xcd/0x2e0
>   shmem_get_folio+0x237/0x580
>   collapse_file+0x247/0x1280
>   hpage_collapse_scan_file+0x26e/0x380
>   khugepaged+0x43b/0x810
>   kthread+0xfb/0x120
>   </TASK>
> 
> Make hpage_collapse_test_exit_or_disable() suspend aware so
> that khugepaaged's scan loops can terminate in a timely manner
> and let system enter the sleep state.
> 

Do we want a Fixes: tag, and maybe backport this to stable kernels?

> Co-developed-by: Baolin Wang <baolin.wang@linux.alibaba.com>
> Signed-off-by: Sergey Senozhatsky <senozhatsky@chromium.org>
> ---
> 
> v1->v2: Actually pass "cc" to hpage_collapse_test_exit_or_disable()
> 
>   mm/khugepaged.c | 22 +++++++++++++++-------
>   1 file changed, 15 insertions(+), 7 deletions(-)
> 
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index eff9e3061925..d32a5ad27097 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -392,10 +392,18 @@ static inline int hpage_collapse_test_exit(struct mm_struct *mm)
>   	return atomic_read(&mm->mm_users) == 0;
>   }
>   
> -static inline int hpage_collapse_test_exit_or_disable(struct mm_struct *mm)
> +static inline int hpage_collapse_test_exit_or_disable(struct mm_struct *mm,
> +						struct collapse_control *cc)

Two-tab indent, please.

>   {
> +	bool was_frozen = false;
> +
> +	if (cc->is_khugepaged &&
> +	    unlikely(kthread_freezable_should_stop(&was_frozen)))
> +		return 1;

I'm trying to understand why kthread_freezable_should_stop() is so 
confusing.

It has this !kthread_should_stop() logic in there, which, IIUC, is not 
really required for the issue here.

But it doesn't hurt to check here whether the kthread is getting shut down.

Relevant for the fix is for us to quit when was_frozen is set, so we can 
end up in khugepaged_wait_work()->wait_event_freezable_timeout().


So using kthread_freezable_should_stop() is fine.


> +
>   	return hpage_collapse_test_exit(mm) ||
> -		mm_flags_test(MMF_DISABLE_THP_COMPLETELY, mm);
> +		mm_flags_test(MMF_DISABLE_THP_COMPLETELY, mm) ||
> +		was_frozen;
>   }


Do we also have to enlighten the kthread_should_stop() check in 
khugepaged_do_scan() to check kthread_freezable_should_stop() instead?

-- 
Cheers,

David