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

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Message-ID: <20180411192448.GD22494@bombadil.infradead.org>
Date:   Wed, 11 Apr 2018 12:24:48 -0700
From:   Matthew Wilcox <willy@...radead.org>
To:     Christopher Lameter <cl@...ux.com>
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, Apr 11, 2018 at 08:44:23AM -0500, Christopher Lameter wrote:
> > +++ b/mm/slub.c
> > @@ -2725,7 +2726,7 @@ static __always_inline void *slab_alloc_node(struct kmem_cache *s,
> >  		stat(s, ALLOC_FASTPATH);
> >  	}
> >
> > -	if (unlikely(gfpflags & __GFP_ZERO) && object)
> > +	if (unlikely(gfpflags & __GFP_ZERO) && object && slab_no_ctor(s))
> >  		memset(object, 0, s->object_size);
> >
> >  	slab_post_alloc_hook(s, gfpflags, 1, &object);
> 
> Please put this in a code path that is enabled by specifying
> 
> slub_debug
> 
> on the kernel command line.

I don't understand.  First, I had:

	if (unlikely(gfpflags & __GFP_ZERO) && object && !WARN_ON_ONCE(s->ctor))

and you didn't like that because it was putting checking into a (semi)fast
path.  Now you want me to add a check for slub_debug somewhere?  I dont
see an existing one I can leverage that will hit on every allocation.
Perhaps I'm missing something.