linux-kernel - Re: [PATCH linux-next v2] ksm: add ksm involvement information for each process

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Date: Fri, 26 Apr 2024 10:13:04 +0200
From: David Hildenbrand <david@...hat.com>
To: xu.xin16@....com.cn, akpm@...ux-foundation.org
Cc: linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
 shr@...kernel.io
Subject: Re: [PATCH linux-next v2] ksm: add ksm involvement information for
 each process

On 26.04.24 03:46, xu.xin16@....com.cn wrote:
> From: xu xin <xu.xin16@....com.cn>
> 
> In /proc/<pid>/ksm_stat, Add two extra ksm involvement items including
> MMF_VM_MERGEABLE and MMF_VM_MERGE_ANY. It helps administrators to
> better know the system's KSM behavior at process level.
> 
> KSM_mergeable: yes/no
> 	whether the process'mm is added by madvise() into the candidate list
> 	of KSM or not.
> KSM_merge_any: yes/no
> 	whether the process'mm is added by prctl() into the candidate list
> 	of KSM or not, and fully enabled at process level.
> 

Thinking about it, we should avoid exposing internal toggles with 
unclear semantics to the user. See below.

> Changelog
> =========
> v1 -> v2:
> 	replace the internal flag names with straightforward strings.
> 	* MMF_VM_MERGEABLE -> KSM_mergeable
> 	* MMF_VM_MERGE_ANY -> KSM_merge_any
> 
> Signed-off-by: xu xin <xu.xin16@....com.cn>
> ---
>   fs/proc/base.c | 4 ++++
>   1 file changed, 4 insertions(+)
> 
> diff --git a/fs/proc/base.c b/fs/proc/base.c
> index 18550c071d71..50e808ffcda4 100644
> --- a/fs/proc/base.c
> +++ b/fs/proc/base.c
> @@ -3217,6 +3217,10 @@ static int proc_pid_ksm_stat(struct seq_file *m, struct pid_namespace *ns,
>   		seq_printf(m, "ksm_zero_pages %lu\n", mm->ksm_zero_pages);
>   		seq_printf(m, "ksm_merging_pages %lu\n", mm->ksm_merging_pages);
>   		seq_printf(m, "ksm_process_profit %ld\n", ksm_process_profit(mm));
> +		seq_printf(m, "KSM_mergeable: %s\n",
> +				test_bit(MMF_VM_MERGEABLE, &mm->flags) ? "yes" : "no");

All it *currently* means is "we called __ksm_enter()" once. It does not 
mean that KSM is still enabled for that process and that any VMA would 
be considered for merging.

I don't think we should expose this.

That information can be more reliably had by looking at

"/proc/pid/smaps" and looking for "mg".

Which tells you exactly if any VMA (and which) is currently applicable 
to KSM.


> +		seq_printf(m, "KSM_merge_any: %s\n",
> +				test_bit(MMF_VM_MERGE_ANY, &mm->flags) ? "yes" : "no");

This makes more sense to export. It's the same as reading 
prctl(PR_GET_MEMORY_MERGE).

The man page [1] calls it simply "KSM has been enabled for this 
process", so process-wide KSM compared to per-VMA KSM.

"KSM_enabled:"

*might* be more reasonable in the context of PR_SET_MEMORY_MERGE.

It wouldn't tell though if KSM is enabled on the system, though.


[1] 
https://lore.kernel.org/linux-mm/20230227220206.436662-1-shr@devkernel.io/T/

-- 
Cheers,

David / dhildenb