Re: [PATCH] mm/zbud: init user ops only when it is needed

[Heesub Shin]

Hello,

On 10/16/2014 05:17 AM, Andrew Morton wrote:
> On Wed, 15 Oct 2014 19:00:43 +0900 Heesub Shin <heesub.shin@xxxxxxxxxxx> wrote:
>
> > When zbud is initialized through the zpool wrapper, pool->ops which
> > points to user-defined operations is always set regardless of whether it
> > is specified from the upper layer. This causes zbud_reclaim_page() to
> > iterate its loop for evicting pool pages out without any gain.
> >
> > This patch sets the user-defined ops only when it is needed, so that
> > zbud_reclaim_page() can bail out the reclamation loop earlier if there
> > is no user-defined operations specified.
>
> Which callsite is calling zbud_zpool_create(..., NULL)?

Currently nowhere. zswap is the only user of zbud and always passes a 
pointer to user-defined operation on pool creation. In addition, there 
may be less possibility that pool shrinking is requested by users who 
did not provide the user-defined ops. So, we may not need to worry much 
about what I wrote in the changelog. However, it is definitely weird to 
pass an argument, zpool_ops, which even will not be referenced by 
zbud_zpool_create(). Above all, it would be more useful to avoid the 
possibility in the future rather than just ignoring it.

regards,
heesub

> > ...
> > --- a/mm/zbud.c
> > +++ b/mm/zbud.c
> > @@ -132,7 +132,7 @@ static struct zbud_ops zbud_zpool_ops = {
> >
> >   static void *zbud_zpool_create(gfp_t gfp, struct zpool_ops *zpool_ops)
> >   {
> > -	return zbud_create_pool(gfp, &zbud_zpool_ops);
> > +	return zbud_create_pool(gfp, zpool_ops ? &zbud_zpool_ops : NULL);
> >   }
> >
> >   static void zbud_zpool_destroy(void *pool)
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