Re: [patch 2/3] memcg: prevent endless loop on huge page charge

From: Johannes Weiner
Date: Thu Jan 27 2011 - 09:15:10 EST

Hi Gleb,

On Thu, Jan 27, 2011 at 04:00:24PM +0200, Gleb Natapov wrote:
> On Thu, Jan 27, 2011 at 02:46:45PM +0100, Johannes Weiner wrote:
> > The charging code can encounter a charge size that is bigger than a
> > regular page in two situations: one is a batched charge to fill the
> > per-cpu stocks, the other is a huge page charge.
> >
> > This code is distributed over two functions, however, and only the
> > outer one is aware of huge pages. In case the charging fails, the
> > inner function will tell the outer function to retry if the charge
> > size is bigger than regular pages--assuming batched charging is the
> > only case. And the outer function will retry forever charging a huge
> > page.
> >
> > This patch makes sure the inner function can distinguish between batch
> > charging and a single huge page charge. It will only signal another
> > attempt if batch charging failed, and go into regular reclaim when it
> > is called on behalf of a huge page.
> >
> Yeah, that is exactly the case I am debugging right now. Came up with
> different solution: pass page_size to __mem_cgroup_do_charge() and
> compare csize with page_size (not CHARGE_SIZE). Not sure which solution
> it more correct.

I guess it makes no difference, but using CHARGE_SIZE gets away
without adding another parameter to __mem_cgroup_do_charge().
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