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Date: Thu, 12 Apr 2018 09:10:23 -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 Wed, 11 Apr 2018, Matthew Wilcox wrote:

>
> I don't see how that works ... can you explain a little more?
>
> I see ___slab_alloc() is called from __slab_alloc().  And I see
> slab_alloc_node does this:
>
>         object = c->freelist;
>         page = c->page;
>         if (unlikely(!object || !node_match(page, node))) {
>                 object = __slab_alloc(s, gfpflags, node, addr, c);
>                 stat(s, ALLOC_SLOWPATH);
>
> But I don't see how slub_debug leads to c->freelist always being NULL.
> It looks like it gets repopulated from page->freelist in ___slab_alloc()
> at the load_freelist label.

c->freelist is NULL and thus ___slab_alloc (slowpath) is called.
___slab_alloc populates c->freelist and gets the new object pointer.

if debugging is on then c->freelist is set to NULL at the end of
___slab_alloc because deactivate_slab() is called.

Thus the next invocation of the fastpath will find that c->freelist is
NULL and go to the slowpath. ...