Re: [RFC] mm/shrinker: define INIT_SHRINKER macro

[Sergey Senozhatsky]

On (07/10/15 15:32), Andrew Morton wrote:
> > Shrinker API does not handle nicely unregister_shrinker() on a not-registered
> > ->shrinker. Looking at shrinker users, they all have to (a) carry on some sort
> > of a flag telling that "unregister_shrinker()" will not blow up... or (b) just
> > be fishy
> > 
> > ...
> >
> > I was thinking of a trivial INIT_SHRINKER macro to init `struct shrinker'
> > internal members (composed in email client, not tested)
> > 
> > include/linux/shrinker.h
> > 
> > #define INIT_SHRINKER(s)			\
> > 	do {					\
> > 		(s)->nr_deferred = NULL;	\
> > 		INIT_LIST_HEAD(&(s)->list);	\
> > 	} while (0)
> 
> Spose so.  Although it would be simpler to change unregister_shrinker()
> to bale out if list.next==NULL and then say "all zeroes is the
> initialized state".

Yes, or '->nr_deferred == NULL' -- we can't have NULL ->nr_deferred
in a properly registered shrinker (as of now)

register_shrinker()
...
        shrinker->nr_deferred = kzalloc(size, GFP_KERNEL);
        if (!shrinker->nr_deferred)
                return -ENOMEM;

        down_write(&shrinker_rwsem);
        list_add_tail(&shrinker->list, &shrinker_list);
        up_write(&shrinker_rwsem);
        return 0;
...


But that will not work if someone has accidentally passed not zeroed
out pointer to unregister.

e.g.

...

	struct foo *bar = kmalloc(..) /* no __GFP_ZERO */

	... something goes wrong and we 'goto err' before
	shrinker_register()

err:
	unregister_shrinker(&bar->shrinker);

...


->list.next and ->nr_deferred won't help us here.
That was the reason to have INIT_SHRINKER/shrinker_init().

But adding an additional check to unregister_shrinker() will not harm.


> > --- a/include/linux/shrinker.h
> > +++ b/include/linux/shrinker.h
> > @@ -63,6 +63,12 @@ struct shrinker {
> >  };
> >  #define DEFAULT_SEEKS 2 /* A good number if you don't know better. */
> >  
> > +#define INIT_SHRINKER(s) 			\
> > +	do {					\
> > +		INIT_LIST_HEAD(&(s)->list);	\
> > +		(s)->nr_deferred = NULL;	\
> > +	} while (0)
> > +
> 
> The only reason to make this a macro would be so that it can be used at
> compile-time, with something like
> 
> static struct shrinker my_shrinker = INIT_SHRINKER(&my_shrinker);
> 
> But as we're not planning on doing that, we implement it in C, please.
> 
> Also, shrinker_init() would be a better name.  Although we already
> mucked up shrinker_register() and shrinker_unregister().
> 

Sure. Will do. Thanks.

	-ss
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