linux-kernel - Re: [patch 8/9] mm: memcontrol: rewrite charge API

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Date:	Fri, 23 May 2014 17:18:52 +0200
From:	Michal Hocko <mhocko@...e.cz>
To:	Johannes Weiner <hannes@...xchg.org>
Cc:	linux-mm@...ck.org, Hugh Dickins <hughd@...gle.com>,
	Tejun Heo <tj@...nel.org>, cgroups@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [patch 8/9] mm: memcontrol: rewrite charge API

On Fri 23-05-14 16:54:13, Michal Hocko wrote:
> On Wed 30-04-14 16:25:42, Johannes Weiner wrote:
> > The memcg charge API charges pages before they are rmapped - i.e. have
> > an actual "type" - and so every callsite needs its own set of charge
> > and uncharge functions to know what type is being operated on.
> > 
> > Rewrite the charge API to provide a generic set of try_charge(),
> > commit_charge() and cancel_charge() transaction operations, much like
> > what's currently done for swap-in:
> > 
> >   mem_cgroup_try_charge() attempts to reserve a charge, reclaiming
> >   pages from the memcg if necessary.
> > 
> >   mem_cgroup_commit_charge() commits the page to the charge once it
> >   has a valid page->mapping and PageAnon() reliably tells the type.
> > 
> >   mem_cgroup_cancel_charge() aborts the transaction.
> > 
> > As pages need to be committed after rmap is established but before
> > they are added to the LRU, page_add_new_anon_rmap() must stop doing
> > LRU additions again.  Factor lru_cache_add_active_or_unevictable().
> > 
> > The order of functions in mm/memcontrol.c is entirely random, so this
> > new charge interface is implemented at the end of the file, where all
> > new or cleaned up, and documented code should go from now on.
> 
> I would prefer moving them after refactoring because the reviewing is
> really harder this way. If such moving is needed at all.
> 
> Anyway this is definitely not a Friday material...
> 
> So only a first impression from a quick glance.
> 
> size is saying the code is slightly bigger:
>    text    data     bss     dec     hex filename
>  487977   84898   45984  618859   9716b mm/built-in.o.7
>  488276   84898   45984  619158   97296 mm/built-in.o.8
> 
> No biggie though.
> 
> It is true it get's rid of ~80LOC in memcontrol.c but it adds some more
> outside of memcg. Most of the charging paths didn't get any easier, they
> already know the type and they have to make sure they even commit the
> charge now.
> 
> But maybe it is just me feeling that now that we have
> mem_cgroup_charge_{anon,file,swapin} the API doesn't look so insane
> anymore and so I am not tempted to change it that much.
> 
> I will look at this with a Monday and fresh brain again.

And now that I got to 9/9 it is obvious it helps a lot to clean up the
uncharge path. But I am not in a mental state to dive into this today.
-- 
Michal Hocko
SUSE Labs
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