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

[Johannes Weiner]

On Tue, Feb 04, 2014 at 08:11:25PM +0100, Michal Hocko wrote:
> 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.

You just have to check ptr=NULL callsites...?  "Callsites that don't
provide their own memcg pass a valid mm pointer for lookup."

Anyway, I'm just telling you what threw me off during review.
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