linux-kernel - Re: [PATCH -v2 2/6] memcg: cleanup charge routines

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Date:	Tue, 4 Feb 2014 20:11:25 +0100
From:	Michal Hocko <mhocko@...e.cz>
To:	Johannes Weiner <hannes@...xchg.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	LKML <linux-kernel@...r.kernel.org>, linux-mm@...ck.org
Subject: Re: [PATCH -v2 2/6] memcg: cleanup charge routines

On Tue 04-02-14 11:40:50, Johannes Weiner wrote:
> On Tue, Feb 04, 2014 at 05:12:30PM +0100, Michal Hocko wrote:
> > On Tue 04-02-14 11:05:09, Johannes Weiner wrote:
> > > On Tue, Feb 04, 2014 at 02:28:56PM +0100, Michal Hocko wrote:
> > > > -	/*
> > > > -	 * We always charge the cgroup the mm_struct belongs to.
> > > > -	 * The mm_struct's mem_cgroup changes on task migration if the
> > > > -	 * thread group leader migrates. It's possible that mm is not
> > > > -	 * set, if so charge the root memcg (happens for pagecache usage).
> > > > -	 */
> > > > -	if (!*ptr && !mm)
> > > > -		*ptr = root_mem_cgroup;
> > > 
> > > [...]
> > > 
> > > >  /*
> > > > + * Charges and returns memcg associated with the given mm (or root_mem_cgroup
> > > > + * if mm is NULL). Returns NULL if memcg is under OOM.
> > > > + */
> > > > +static struct mem_cgroup *mem_cgroup_try_charge_mm(struct mm_struct *mm,
> > > > +				   gfp_t gfp_mask,
> > > > +				   unsigned int nr_pages,
> > > > +				   bool oom)
> > > > +{
> > > > +	struct mem_cgroup *memcg;
> > > > +	int ret;
> > > > +
> > > > +	/*
> > > > +	 * We always charge the cgroup the mm_struct belongs to.
> > > > +	 * The mm_struct's mem_cgroup changes on task migration if the
> > > > +	 * thread group leader migrates. It's possible that mm is not
> > > > +	 * set, if so charge the root memcg (happens for pagecache usage).
> > > > +	 */
> > > > +	if (!mm)
> > > > +		goto bypass;
> > > 
> > > Why shuffle it around right before you remove it anyway?  Just start
> > > the series off with the patches that delete stuff without having to
> > > restructure anything, get those out of the way.
> > 
> > As mentioned in the previous email. I wanted to have this condition
> > removal bisectable. So it is removed in the next patch when it is
> > replaced by VM_BUG_ON.
> 
> I'm not suggesting to sneak the removal into *this* patch,

OK

> just put the simple stand-alone patches that remove stuff first in the
> series.

In this particular case, though, the reduced condition is much easier
to review IMO. Just look at the jungle of different *ptr vs. mm
combinations described in this patch description which would have to be
reviewed separately if I moved the removal before this patch.
The ptr part of the original condition went away naturally here while
the reasoning why there is no code path implicitly relying on (!ptr &&
!mm) resulting in bypass would be harder.

> Seems pretty logical to me to first reduce the code base as much as
> possible before reorganizing it.  This does not change bisectability
> but it sure makes the patches easier to read.

Agreed.
-- 
Michal Hocko
SUSE Labs
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