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

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Message-ID: <20150126204318.GB3317@esperanza>
Date:	Mon, 26 Jan 2015 23:43:18 +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 02:28:33PM -0600, Christoph Lameter wrote:
> On Mon, 26 Jan 2015, Vladimir Davydov wrote:
> 
> > 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.
> 
> The acpi subsystem did that at some point.
> 
> > 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?
> 
> That is up to the subsystem. If it has a means of tracking down the
> missing object then it can deal with it. If not then it cannot shutdown
> the cache and do a proper recovery action.

Hmm, we could make kmem_cache_destroy return EBUSY for the purpose.
However, since it spits warnings on failure, which is reasonable, we
have this check in kmem_cache_shrink...

Anyways, I see your point now, thank you for pointing it out. I will fix
SLUB's __kmem_cache_shrink retval instead of removing it altogether in
the next iteration.

Thanks,
Vladimir
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