linux-kernel - Re: [RFC] mm: show message when updating min_free

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Message-Id: <20140114163533.ab191e118e82ca7b4d499551@linux-foundation.org>
Date:	Tue, 14 Jan 2014 16:35:33 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	David Rientjes <rientjes@...gle.com>
Cc:	Han Pingtian <hanpt@...ux.vnet.ibm.com>,
	linux-kernel@...r.kernel.org, Michal Hocko <mhocko@...e.cz>,
	Mel Gorman <mgorman@...e.de>, linux-mm@...ck.org,
	Dave Hansen <dave.hansen@...el.com>
Subject: Re: [RFC] mm: show message when updating min_free_kbytes in thp

On Tue, 14 Jan 2014 16:25:10 -0800 (PST) David Rientjes <rientjes@...gle.com> wrote:

> On Tue, 14 Jan 2014, Andrew Morton wrote:
> 
> > 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.
> > 
> 
> I can vouch for kernel logs that are spammed with tons of net page 
> allocation failure warnings, in fact we're going through and adding 
> __GFP_NOWARN to most of these.
> 
> > 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.
> > 
> 
> Indeed, we have initscripts that modified min_free_kbytes before thp was 
> introduced but luckily they were comparing their newly computed value to 
> the existing value and only writing if the new value is greater.  
> Hopefully most users are doing the same thing.

I've been waiting 10+ years for us to decide to delete that warning due
to the false positives.  Hasn't happened yet, and the warning does
find bugs/issues/misconfigurations/etc.

But I do worry this has led to users unnecessarily increasing
min_free_kbytes just to shut the warnings up.  Net result: they have
less memory available for cache, etc.

> Would it be overkill to save the kernel default both with and without thp 
> and then doing a WARN_ON_ONCE() if a user-written value is ever less?

Well, min_free_kbytes is a userspace thing, not a kernel thing - maybe
THP shouldn't be dinking with it.  What effect is THP trying to achieve
and can we achieve it by other/better means?
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