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Date:   Wed, 9 May 2018 23:00:09 +0200
From:   Michal Hocko <mhocko@kernel.org>
To:     Andrew Morton <akpm@linux-foundation.org>
Cc:     kbuild test robot <lkp@intel.com>,
        "Eric W. Biederman" <ebiederm@xmission.com>, kbuild-all@01.org,
        Johannes Weiner <hannes@cmpxchg.org>,
        Kirill Tkhai <ktkhai@virtuozzo.com>, peterz@infradead.org,
        oleg@redhat.com, viro@zeniv.linux.org.uk, mingo@kernel.org,
        paulmck@linux.vnet.ibm.com, keescook@chromium.org, riel@redhat.com,
        tglx@linutronix.de, kirill.shutemov@linux.intel.com,
        marcos.souza.org@gmail.com, hoeun.ryu@gmail.com,
        pasha.tatashin@oracle.com, gs051095@gmail.com, dhowells@redhat.com,
        rppt@linux.vnet.ibm.com, linux-kernel@vger.kernel.org,
        Balbir Singh <balbir@linux.vnet.ibm.com>,
        Tejun Heo <tj@kernel.org>
Subject: Re: [PATCH] memcg: Replace mm->owner with mm->memcg
Message-ID: <20180509210009.GW32366@dhcp22.suse.cz>
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On Mon 07-05-18 16:18:01, Andrew Morton wrote:
> On Sun, 6 May 2018 00:54:48 +0800 kbuild test robot <lkp@intel.com> wrote:
> 
> > Hi Eric,
> > 
> > Thank you for the patch! Yet something to improve:
> > 
> > [auto build test ERROR on linus/master]
> > [also build test ERROR on v4.17-rc3]
> > [cannot apply to next-20180504]
> > [if your patch is applied to the wrong git tree, please drop us a note to help improve the system]
> > 
> > url:    https://github.com/0day-ci/linux/commits/Eric-W-Biederman/memcg-Replace-mm-owner-with-mm-memcg/20180503-120054
> > config: xtensa-nommu_kc705_defconfig (attached as .config)
> > compiler: xtensa-de212-elf-gcc (crosstool-NG crosstool-ng-1.23.0-307-g452ee331) 7.3.0
> > reproduce:
> >         wget https://raw.githubusercontent.com/intel/lkp-tests/master/sbin/make.cross -O ~/bin/make.cross
> >         chmod +x ~/bin/make.cross
> >         # save the attached .config to linux build tree
> >         make.cross ARCH=xtensa 
> > 
> > All errors (new ones prefixed by >>):
> > 
> >    kernel/fork.o: In function `sighand_ctor':
> > >> fork.c:(.text+0x3b0): undefined reference to `mm_update_memcg'
> >    kernel/fork.o: In function `mmdrop_async':
> >    fork.c:(.text+0x43a): undefined reference to `mm_update_memcg'
> >    kernel/fork.o: In function `set_task_stack_end_magic':
> >    fork.c:(.text+0x752): undefined reference to `mm_update_memcg'
> 
> Due to the combination of CONFIG_MMU=n and CONFIG_MEMCG=y. 
> mm/memcontrol.c's mm_update_memcg() depends upon CONFIG_MMU=y.
> 
> Is this a valid conbination?  Do we actually support (and test!) memcg
> on nommu systems?

Well, I was arguing we should mark it BROKEN quite some time ago but
Johannes and you IIRC objected that we shouldn't do that as long as
nobody complains it doesn't work. To be completely honest I am skeptical
anybody uses memcg on nommu, but I might be easily wrong here. I didn't
care enough to pursue back then...

-- 
Michal Hocko
SUSE Labs