linux-kernel - Re: [PATCH -mm 2/3] slab: zap kmem_cache

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Message-ID: <20150126201602.GA3317@esperanza>
Date:	Mon, 26 Jan 2015 23:16:02 +0300
From:	Vladimir Davydov <vdavydov@...allels.com>
To:	Christoph Lameter <cl@...ux.com>
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	Pekka Enberg <penberg@...nel.org>,
	David Rientjes <rientjes@...gle.com>,
	Joonsoo Kim <iamjoonsoo.kim@....com>,
	Johannes Weiner <hannes@...xchg.org>,
	Michal Hocko <mhocko@...e.cz>, <linux-mm@...ck.org>,
	<linux-kernel@...r.kernel.org>
Subject: Re: [PATCH -mm 2/3] slab: zap kmem_cache_shrink return value

On Mon, Jan 26, 2015 at 01:55:14PM -0600, Christoph Lameter wrote:
> On Mon, 26 Jan 2015, Vladimir Davydov wrote:
> 
> > Hmm, why? The return value has existed since this function was
> > introduced, but nobody seems to have ever used it outside the slab core.
> > Besides, this check is racy, so IMO we shouldn't encourage users of the
> > API to rely on it. That said, I believe we should drop the return value
> > for now. If anybody ever needs it, we can reintroduce it.
> 
> The check is only racy if you have concurrent users. It is not racy if a
> subsystem shuts down access to the slabs and then checks if everything is
> clean before closing the cache.
>
> Slab creation and destruction are not serialized. It is the responsibility
> of the subsystem to make sure that there are no concurrent users and that
> there are no objects remaining before destroying a slab.

Right, but I just don't see why a subsystem using a kmem_cache would
need to check whether there are any objects left in the cache. I mean,
it should somehow keep track of the objects it's allocated anyway, e.g.
by linking them in a list. That means it must already have a way to
check if it is safe to destroy its cache or not.

Suppose we leave the return value as is. A subsystem, right before going
to destroy a cache, calls kmem_cache_shrink, which returns 1 (slab is
not empty). What is it supposed to do then?

Thanks,
Vladimir
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