Re: [RFC] mm: show message when updating min_free_kbytes in thp

[Andrew Morton]

On Wed, 15 Jan 2014 04:07:20 +0800 Han Pingtian <hanpt@xxxxxxxxxxxxxxxxxx> wrote:

> min_free_kbytes may be raised during THP's initialization. Sometimes,
> this will change the value being set by user. Showing message will
> clarify this confusion.
> 
> Only show this message when changing the value set by user according to
> Michal Hocko's suggestion.
> 
> Showing the old value of min_free_kbytes according to Dave Hansen's
> suggestion. This will give user the chance to restore old value of
> min_free_kbytes.
> 

This is all a bit nasty, isn't it?  THP goes and alters min_free_kbytes
to improve its own reliability, but min_free_kbytes is also
user-modifiable.  And over many years we have trained a *lot* of users
to alter min_free_kbytes.  Often to prevent nasty page allocation
failure warnings from net drivers.

So there are probably quite a lot of people out there who are manually
rubbing out THP's efforts.  And there may also be people who are
setting min_free_kbytes to a value which is unnecessarily high for more
recent kernels.

I don't know what to do about this mess though :(

> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -130,8 +130,14 @@ static int set_recommended_min_free_kbytes(void)
>  			      (unsigned long) nr_free_buffer_pages() / 20);
>  	recommended_min <<= (PAGE_SHIFT-10);
>  
> -	if (recommended_min > min_free_kbytes)
> +	if (recommended_min > min_free_kbytes) {
> +		if (user_min_free_kbytes >= 0)
> +			pr_info("raising min_free_kbytes from %d to %lu "
> +				"to help transparent hugepage allocations\n",
> +				min_free_kbytes, recommended_min);

hm, recommended_min shouldn't have had long type.  Oh well, we've done
worse things.

>  		min_free_kbytes = recommended_min;
> +	}
>  	setup_per_zone_wmarks();
>  	return 0;
>  }

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