linux-kernel - RE: [PATCH] slab, memcontrol: undefined reference to `memcg_kmem_get

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Date:   Tue, 24 Sep 2019 14:08:33 +0000
From:   Mircea CIRJALIU - MELIU <mcirjaliu@...defender.com>
To:     Michal Hocko <mhocko@...nel.org>
CC:     "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "cl@...ux.com" <cl@...ux.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH] slab, memcontrol: undefined reference to
 `memcg_kmem_get_cache'

> On Tue 24-09-19 08:46:48, Mircea CIRJALIU - MELIU wrote:
> > Having CONFIG_MEMCG turned off causes these issues:
> > 	mm/slub.o: In function `slab_pre_alloc_hook':
> > 	/home/mircea/build/mm/slab.h:425: undefined reference to
> `memcg_kmem_get_cache'
> > 	mm/slub.o: In function `slab_post_alloc_hook':
> > 	/home/mircea/build/mm/slab.h:444: undefined reference to
> `memcg_kmem_put_cache'
> 
> You should be adding your Sign-off-by to every patch you post to the kernel
> mailing list (see Documentation/SubmittingPatches).
> 
> It is also really important to mention which tree does this apply to and ideally
> also note which change has broken the code. In this particular case I have
> tried the current Linus tree (4c07e2ddab5b) and $ grep
> 'CONFIG_SLUB\|CONFIG_MEMCG' .config # CONFIG_MEMCG is not set
> CONFIG_SLUB_DEBUG=y CONFIG_SLUB=y CONFIG_SLUB_CPU_PARTIAL=y
> # CONFIG_SLUB_DEBUG_ON is not set # CONFIG_SLUB_STATS is not set
> 
> which means CONFIG_MEMCG_KMEM is not enabled as well. And the
> compilation succeeds. What is your config file?

The config file is not the problem (figured it out).
I am lowering the optimization level on certain files for debug purposes.
In my case: CFLAGS_slub.o += -O1 -fno-inline

The code which causes the problem looks like this:
static inline struct kmem_cache *slab_pre_alloc_hook(struct kmem_cache *s,
						     gfp_t flags)
{
	...

	if (memcg_kmem_enabled() &&
	    ((flags & __GFP_ACCOUNT) || (s->flags & SLAB_ACCOUNT)))
		return memcg_kmem_get_cache(s);

	...
}

Under normal circumstances memcg_kmem_enabled() returns false and the
statement is evaluated as unreachable and removed entirely.
It seems -O1 keeps the call to memcg_kmem_get_cache().

The change that introduced this is here: 
	commit 452647784b2fccfdeeb976f6f842c6719fb2daac
	Author: Vladimir Davydov <vdavydov@...tuozzo.com>
	Date:   Tue Jul 26 15:24:21 2016 -0700
Although I had the same problem before with other header files.