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SubjectRe: [patch 2/3] memcg: prevent endless loop when charging huge pages to near-limit group
On Mon, Jan 31, 2011 at 02:41:31PM -0800, Andrew Morton wrote:
> On Mon, 31 Jan 2011 15:03:54 +0100
> Johannes Weiner <hannes@cmpxchg.org> wrote:
> > @@ -1111,6 +1111,15 @@ static bool mem_cgroup_check_under_limit(struct mem_cgroup *mem)
> > return false;
> > }
> >
> > +static bool mem_cgroup_check_margin(struct mem_cgroup *mem, unsigned long bytes)
> > +{
> > + if (!res_counter_check_margin(&mem->res, bytes))
> > + return false;
> > + if (do_swap_account && !res_counter_check_margin(&mem->memsw, bytes))
> > + return false;
> > + return true;
> > +}
>
> argh.
>
> If you ever have a function with the string "check" in its name, it's a
> good sign that you did something wrong.
>
> Check what? Against what? Returning what?
>
> mem_cgroup_check_under_limit() isn't toooo bad - the name tells you
> what's being checked and tells you what to expect the return value to
> mean.
>
> But "res_counter_check_margin" and "mem_cgroup_check_margin" are just
> awful. Something like
>
> bool res_counter_may_charge(counter, bytes)
>
> would be much clearer.

That makes sense for the hard limit. But the oh-so-generic resource
counters also have a soft limit, and you don't ask for that when you
want to charge. Right now, I do not feel creative enough to come up
with a symmetric-sounding counterpart.

> If we really want to stick with the "check" names (perhaps as an ironic
> reference to res_counter's past mistakes) then please at least document
> the sorry things?

I cowardly went with this option and have a patch below to fold into
this fix.

Maybe it would be better to use res_counter_margin(cnt) >= wanted
throughout the code. Or still better, make memcg work on pages and
res_counters on unsigned longs so the locking is no longer needed,
together with an API for most obvious maths. I will work something
out and submit it separately.

---
Subject: [patch fixup] res_counter: document res_counter_check_margin()
Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
---
diff --git a/include/linux/res_counter.h b/include/linux/res_counter.h
index 5cfd78a..a5930cb 100644
--- a/include/linux/res_counter.h
+++ b/include/linux/res_counter.h
@@ -182,6 +182,14 @@ static inline bool res_counter_check_under_limit(struct res_counter *cnt)
return ret;
}

+/**
+ * res_counter_check_margin - check if the counter allows charging
+ * @cnt: the resource counter to check
+ * @bytes: the number of bytes to check the remaining space against
+ *
+ * Returns a boolean value on whether the counter can be charged
+ * @bytes or whether this would exceed the limit.
+ */
static inline bool res_counter_check_margin(struct res_counter *cnt,
unsigned long bytes)
{

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