compaction: trying to understand the code

From: "Iram Shahzad" <iram.shahzad@jp.fujitsu.com>
To: Mel Gorman <mel@csn.ul.ie>
Cc: linux-mm@kvack.org
Subject: compaction: trying to understand the code
Date: Tue, 17 Aug 2010 20:08:54 +0900	[thread overview]
Message-ID: <325E0A25FE724BA18190186F058FF37E@rainbow> (raw)

Hi

I am trying to understand the following code in isolate_migratepages
function. I have a question regarding this.

---
 while (unlikely(too_many_isolated(zone))) {
  congestion_wait(BLK_RW_ASYNC, HZ/10);

  if (fatal_signal_pending(current))
   return 0;
 }

---

I have seen that in some cases this while loop never exits
because too_many_isolated keeps returning true for ever.
And hence the process hangs. Is this intended behaviour?
What is it that is supposed to change the "too_many_isolated" situation?
In other words, what is it that is supposed to increase the "inactive"
or decrease the "isolated" so that isolated > inactive becomes false?

Best regards
Iram

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