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

From: Matthew Wilcox
Date: Wed Apr 11 2018 - 15:25:02 EST


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.