linux-kernel - Re: [RFC] mm/shrinker: define INIT

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Message-ID: <20150711014812.GD811@swordfish>
Date:	Sat, 11 Jul 2015 10:48:12 +0900
From:	Sergey Senozhatsky <sergey.senozhatsky@...il.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
	Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Minchan Kim <minchan@...nel.org>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [RFC] mm/shrinker: define INIT_SHRINKER macro

On (07/10/15 18:33), Andrew Morton wrote:
> > > > 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)
> 
> list.next seems safer because that will always be non-zero.  But
> whatever - we can change it later.
>  
> > But that will not work if someone has accidentally passed not zeroed
> > out pointer to unregister.
> 
> I wouldn't worry about that really.  If you pass a pointer to
> uninitialized memory, the kernel will explode.  That's true of just
> about every pointer-accepting function in the kernel.
>

True. But with shrinker it's hard to say whether we have a properly
initialized shrinker embedded in our `struct foo' or we don't (unless
we treat register_shrinker() errors as a show stopper) by simply looking at
shrinker struct (w/o touching it's private members). In zsmalloc, for
instance, we don't consider failed register_shrinker() to be critical
enough to forbid zs_pool creation and usage. It makes things harder later
in zs_destroy_pool(), because we need to carry some sort of flag for that
purpose. But `list.next' check in unregister_shrinker() would suffice in
zsmalloc case, I must admit, because we kzalloc() the entire zs_pool
struct.

	-ss
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