On Mon, Jan 31, 2011 at 02:41:31PM -0800, Andrew Morton wrote:

On Mon, 31 Jan 2011 15:03:54 +0100
Johannes Weiner [off-list ref] wrote:

quoted

@@ -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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