Re: [RFC PATCH 3/3] mm, proc: report PR_SET_THP_DISABLE in proc

[Michal Hocko <mhocko@kernel.org>]

Damn, David somehow didn't make it to the CC list. Sorry about that.

On Tue 20-11-18 11:35:15, Michal Hocko wrote:
> From: Michal Hocko <mhocko@suse.com>
> 
> David Rientjes has reported that 1860033237d4 ("mm: make
> PR_SET_THP_DISABLE immediately active") has changed the way how
> we report THPable VMAs to the userspace. Their monitoring tool is
> triggering false alarms on PR_SET_THP_DISABLE tasks because it considers
> an insufficient THP usage as a memory fragmentation resp. memory
> pressure issue.
> 
> Before the said commit each newly created VMA inherited VM_NOHUGEPAGE
> flag and that got exposed to the userspace via /proc/<pid>/smaps file.
> This implementation had its downsides as explained in the commit message
> but it is true that the userspace doesn't have any means to query for
> the process wide THP enabled/disabled status.
> 
> PR_SET_THP_DISABLE is a process wide flag so it makes a lot of sense
> to export in the process wide context rather than per-vma. Introduce
> a new field to /proc/<pid>/status which export this status.  If
> PR_SET_THP_DISABLE is used then it reports false same as when the THP is
> not compiled in. It doesn't consider the global THP status because we
> already export that information via sysfs
> 
> Fixes: 1860033237d4 ("mm: make PR_SET_THP_DISABLE immediately active")
> Signed-off-by: Michal Hocko <mhocko@suse.com>
> ---
>  Documentation/filesystems/proc.txt |  3 +++
>  fs/proc/array.c                    | 10 ++++++++++
>  2 files changed, 13 insertions(+)
> 
> diff --git a/Documentation/filesystems/proc.txt b/Documentation/filesystems/proc.txt
> index 06562bab509a..7995e9322889 100644
> --- a/Documentation/filesystems/proc.txt
> +++ b/Documentation/filesystems/proc.txt
> @@ -182,6 +182,7 @@ For example, to get the status information of a process, all you have to do is
>    VmSwap:        0 kB
>    HugetlbPages:          0 kB
>    CoreDumping:    0
> +  THP_enabled:	  1
>    Threads:        1
>    SigQ:   0/28578
>    SigPnd: 0000000000000000
> @@ -256,6 +257,8 @@ Table 1-2: Contents of the status files (as of 4.8)
>   HugetlbPages                size of hugetlb memory portions
>   CoreDumping                 process's memory is currently being dumped
>                               (killing the process may lead to a corrupted core)
> + THP_enabled		     process is allowed to use THP (returns 0 when
> +			     PR_SET_THP_DISABLE is set on the process
>   Threads                     number of threads
>   SigQ                        number of signals queued/max. number for queue
>   SigPnd                      bitmap of pending signals for the thread
> diff --git a/fs/proc/array.c b/fs/proc/array.c
> index 0ceb3b6b37e7..9d428d5a0ac8 100644
> --- a/fs/proc/array.c
> +++ b/fs/proc/array.c
> @@ -392,6 +392,15 @@ static inline void task_core_dumping(struct seq_file *m, struct mm_struct *mm)
>  	seq_putc(m, '\n');
>  }
>  
> +static inline void task_thp_status(struct seq_file *m, struct mm_struct *mm)
> +{
> +	bool thp_enabled = IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE);
> +
> +	if (thp_enabled)
> +		thp_enabled = !test_bit(MMF_DISABLE_THP, &mm->flags);
> +	seq_printf(m, "THP_enabled:\t%d\n", thp_enabled);
> +}
> +
>  int proc_pid_status(struct seq_file *m, struct pid_namespace *ns,
>  			struct pid *pid, struct task_struct *task)
>  {
> @@ -406,6 +415,7 @@ int proc_pid_status(struct seq_file *m, struct pid_namespace *ns,
>  	if (mm) {
>  		task_mem(m, mm);
>  		task_core_dumping(m, mm);
> +		task_thp_status(m, mm);
>  		mmput(mm);
>  	}
>  	task_sig(m, task);
> -- 
> 2.19.1

-- 
Michal Hocko
SUSE Labs