Re: [PATCH] slub: do not merge cache if slub_debug contains a never-merge flag

[Grygorii Maistrenko <grygoriimkd@gmail.com>]

On Fri, Dec 23, 2016 at 12:30:02PM -0600, Christoph Lameter wrote:
> 
> On Fri, 23 Dec 2016, Grygorii Maistrenko wrote:
> 
> > In case CONFIG_SLUB_DEBUG_ON=n find_mergeable() gets debug features
> > from commandline but never checks if there are features from the
> > SLAB_NEVER_MERGE set.
> > As a result selected by slub_debug caches are always mergeable if they
> > have been created without a custom constructor set or without one of the
> > SLAB_* debug features on.
> 
> WTF is this nonsense? That check is done a few lines earlier!
> 
> struct kmem_cache *ind_mergeable(size_t size, size_t align,
>                 unsigned long flags, const char *name, void (*ctor)(void *))
> {
>         struct kmem_cache *s;
> 
>         if (slab_nomerge || (flags & SLAB_NEVER_MERGE))    <----- !!!!!!
>                 return NULL;

This one check is done on flags passed to kmem_cache_create().

> 
>         if (ctor)
>                 return NULL;
> 
>         size = ALIGN(size, sizeof(void *));
>         align = calculate_alignment(flags,
	flags = kmem_cache_flags(size, flags, name, NULL);

I added here the missing line. This updates flags from commandline and
after this we do not check it.

> 
> 
> >
> > This adds the necessary check and makes selected slab caches unmergeable
> > if one of the SLAB_NEVER_MERGE features is set from commandline.
> >
> > Signed-off-by: Grygorii Maistrenko <grygoriimkd@gmail.com>
> > ---
> >  mm/slab_common.c | 3 +++
> >  1 file changed, 3 insertions(+)
> >
> > diff --git a/mm/slab_common.c b/mm/slab_common.c
> > index 329b03843863..7341cba8c58b 100644
> > --- a/mm/slab_common.c
> > +++ b/mm/slab_common.c
> > @@ -266,6 +266,9 @@ struct kmem_cache *find_mergeable(size_t size, size_t align,
> >  	size = ALIGN(size, align);
> >  	flags = kmem_cache_flags(size, flags, name, NULL);
> >
> > +	if (flags & SLAB_NEVER_MERGE)
> > +		return NULL;
> > +
> >  	list_for_each_entry_reverse(s, &slab_caches, list) {
> >  		if (slab_unmergeable(s))
> >  			continue;
> >

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