Re: 3.13-rc breaks MEMCG_SWAP

[Michal Hocko]

On Mon 16-12-13 17:36:09, Li Zefan wrote:
> On 2013/12/16 16:36, Hugh Dickins wrote:
> > CONFIG_MEMCG_SWAP is broken in 3.13-rc.  Try something like this:
> > 
> > mkdir -p /tmp/tmpfs /tmp/memcg
> > mount -t tmpfs -o size=1G tmpfs /tmp/tmpfs
> > mount -t cgroup -o memory memcg /tmp/memcg
> > mkdir /tmp/memcg/old
> > echo 512M >/tmp/memcg/old/memory.limit_in_bytes
> > echo $$ >/tmp/memcg/old/tasks
> > cp /dev/zero /tmp/tmpfs/zero 2>/dev/null
> > echo $$ >/tmp/memcg/tasks
> > rmdir /tmp/memcg/old
> > sleep 1	# let rmdir work complete
> > mkdir /tmp/memcg/new
> > umount /tmp/tmpfs
> > dmesg | grep WARNING
> > rmdir /tmp/memcg/new
> > umount /tmp/memcg
> > 
> > Shows lots of WARNING: CPU: 1 PID: 1006 at kernel/res_counter.c:91
> >                            res_counter_uncharge_locked+0x1f/0x2f()
> > 
> > Breakage comes from 34c00c319ce7 ("memcg: convert to use cgroup id").
> > 
> > The lifetime of a cgroup id is different from the lifetime of the
> > css id it replaced: memsw's css_get()s do nothing to hold on to the
> > old cgroup id, it soon gets recycled to a new cgroup, which then
> > mysteriously inherits the old's swap, without any charge for it.
> > (I thought memsw's particular need had been discussed and was
> > well understood when 34c00c319ce7 went in, but apparently not.)
> > 
> > The right thing to do at this stage would be to revert that and its
> > associated commits; but I imagine to do so would be unwelcome to
> > the cgroup guys, going against their general direction; and I've
> > no idea how embedded that css_id removal has become by now.
> > 
> > Perhaps some creative refcounting can rescue memsw while still
> > using cgroup id?
> > 
> 
> Sorry for the broken.
> 
> I think we can keep the cgroup->id until the last css reference is
> dropped and the css is scheduled to be destroyed.

How would this work? The task which pushed the memory to the swap is
still alive (living in a different group) and the swap will be there
after the last reference to css as well.
 
> I'll cook a fix tomorrow.
> 
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-- 
Michal Hocko
SUSE Labs
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