Re: [PATCH] mm: clarify COMPACTION Kconfig text

[David Rientjes <rientjes@google.com>]

On Tue, 23 Aug 2016, Michal Hocko wrote:

> From: Michal Hocko <mhocko@suse.com>
> 
> The current wording of the COMPACTION Kconfig help text doesn't
> emphasise that disabling COMPACTION might cripple the page allocator
> which relies on the compaction quite heavily for high order requests and
> an unexpected OOM can happen with the lack of compaction. Make sure
> we are vocal about that.
> 

Since when has this been an issue?  I don't believe it has been an issue 
in the past for any archs that don't use thp.

> Signed-off-by: Michal Hocko <mhocko@suse.com>
> ---
>  mm/Kconfig | 9 ++++++++-
>  1 file changed, 8 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/Kconfig b/mm/Kconfig
> index 78a23c5c302d..0dff2f05b6d1 100644
> --- a/mm/Kconfig
> +++ b/mm/Kconfig
> @@ -262,7 +262,14 @@ config COMPACTION
>  	select MIGRATION
>  	depends on MMU
>  	help
> -	  Allows the compaction of memory for the allocation of huge pages.
> +          Compaction is the only memory management component to form
> +          high order (larger physically contiguous) memory blocks
> +          reliably. Page allocator relies on the compaction heavily and
> +          the lack of the feature can lead to unexpected OOM killer
> +          invocation for high order memory requests. You shouldnm't
> +          disable this option unless there is really a strong reason for
> +          it and then we are really interested to hear about that at
> +          linux-mm@kvack.org.
>  
>  #
>  # support for page migration

This seems to strongly suggest that all kernels should be built with 
CONFIG_COMPACTION and its requirement, CONFIG_MIGRATION.  Migration has a 
dependency of NUMA or memory hot-remove (not all popular).  Compaction can 
defragment memory within single zone without reliance on NUMA.

This seems like a very bizarre requirement and I'm wondering where we 
regressed from this thp-only behavior.

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