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Date: Thu, 12 Apr 2018 10:15:42 -0500 (CDT)
From: Christopher Lameter <cl@linux.com>
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To: Matthew Wilcox <willy@infradead.org>
cc: linux-mm@kvack.org, Matthew Wilcox <mawilcox@microsoft.com>,
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        linux-kernel@vger.kernel.org, Jan Kara <jack@suse.cz>,
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        Mel Gorman <mgorman@techsingularity.net>
Subject: Re: [PATCH v2 2/2] slab: __GFP_ZERO is incompatible with a
 constructor
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On Thu, 12 Apr 2018, Matthew Wilcox wrote:

> > Thus the next invocation of the fastpath will find that c->freelist is
> > NULL and go to the slowpath. ...
>
> _ah_.  I hadn't figured out that c->page was always NULL in the debugging
> case too, so ___slab_alloc() always hits the 'new_slab' case.  Thanks!

Also note that you can have SLUB also do the build with all debugging on
without having to use a command like parameter (like SLAB). That requires
CONFIG_SLUB_DEBUG_ON to be set. CONFIG_SLUB_DEBUG is set by default for
all distro builds. It only includes the debug code but does not activate
them by default. Kernel command line parameters allow you to selectively
switch on debugging features for specific slab caches so that you can
limit the latency variations introduced by debugging into the production
kernel. Thus subtle races may be found.