linux-kernel - Re: [PATCH v2 2/2] slab: __GFP_ZERO is incompatible with a constructor

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Date:   Thu, 12 Apr 2018 09:10:23 -0500 (CDT)
From:   Christopher Lameter <cl@...ux.com>
To:     Matthew Wilcox <willy@...radead.org>
cc:     linux-mm@...ck.org, Matthew Wilcox <mawilcox@...rosoft.com>,
        Pekka Enberg <penberg@...nel.org>,
        David Rientjes <rientjes@...gle.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org, Jan Kara <jack@...e.cz>,
        Jeff Layton <jlayton@...hat.com>,
        Mel Gorman <mgorman@...hsingularity.net>
Subject: Re: [PATCH v2 2/2] slab: __GFP_ZERO is incompatible with a
 constructor

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. ...