linux-kernel - Re: [PATCH] mm: clarify COMPACTION Kconfig text

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Message-ID: <20160823091726.GK23577@dhcp22.suse.cz>
Date:   Tue, 23 Aug 2016 11:17:26 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Markus Trippelsdorf <markus@...ppelsdorf.de>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Mel Gorman <mgorman@...e.de>, Joonsoo Kim <js1304@...il.com>,
        Vlastimil Babka <vbabka@...e.cz>, linux-mm@...ck.org,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mm: clarify COMPACTION Kconfig text

On Tue 23-08-16 10:38:30, Markus Trippelsdorf wrote:
> On 2016.08.23 at 10:09 +0200, Michal Hocko wrote:
> > From: Michal Hocko <mhocko@...e.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.
> 
> Just a few nitpicks inline below:
> 
> >  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 page allo...      on compaction    
> > +          the lack of the feature can lead to unexpected OOM killer
> > +          invocation for high order memory requests. You shouldnm't
>              invocations                                    shouldn't  
> > +          disable this option unless there is really a strong reason for
>                                               really is      
> > +          it and then we are really interested to hear about that at
>                             would be    

Thanks for the review. Updated patch follows:
---
>From 177212367432e9fa5e090fafdfe3774d5793e46b Mon Sep 17 00:00:00 2001
From: Michal Hocko <mhocko@...e.com>
Date: Tue, 23 Aug 2016 10:03:37 +0200
Subject: [PATCH] mm: clarify COMPACTION Kconfig text

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.

Signed-off-by: Michal Hocko <mhocko@...e.com>
---
 mm/Kconfig | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/mm/Kconfig b/mm/Kconfig
index 78a23c5c302d..be0ee11fa0d9 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. The page allocator relies on compaction heavily and
+          the lack of the feature can lead to unexpected OOM killer
+          invocations for high order memory requests. You shouldn't
+          disable this option unless there really is a strong reason for
+          it and then we would be really interested to hear about that at
+          linux-mm@...ck.org.
 
 #
 # support for page migration
-- 
2.8.1

-- 
Michal Hocko
SUSE Labs