On Mon, May 30, 2011 at 05:55:46PM +0100, Mel Gorman wrote: > Even with drift issues, -1 there should be "impossible". Assuming this > is a zoneinfo file, that figure is based on global_page_state() which > looks like The two cases reproducing this long hang in D state, had from SMP=n PREEMPT=y. Clearly not common config these days. Also it didn't seem apparent that any task was running in a code path that kept pages isolated. > unsigned long, and callers are using unsigned long, is there any > possibility the "if (x < 0)" is being optimised out? If you aware It was eliminated by cpp. > of users reporting this problem (like the users in thread "iotop: > khugepaged at 99.99% (2.6.38.3)"), do you know if they had a particular > compiler in common? I had no reason to worry about the compiler yet but that's always good idea to keep in mind. The thread were the bug is reported is the "iotop" one you mentioned, and there's a tarball attached to one of the last emails of the thread with the debug data I grepped. It was /proc/zoneinfo file yes. That's the file I asked when I noticed something had to be wrong with too_many_isolated and I expected either nr_isolated or nr_inactive going wrong, it turned out it was nr_isolated (apparently, I don't have full picture on the problem yet). I added you in CC to a few emails but you weren't in all replies. The debug data you can find on lkml in this email: Message-Id: <201105232005.56840.johannes.hirte@xxxxxxxxxxxxxxxxx>. The other relevant sysrq+t here http://pastebin.com/raw.php?i=VG28YRbi better save the latter (I did) as I'm worried it has a timeout on it. Your patch was for reports with CONFIG_SMP=y? I'd prefer to clear out this error before improving the too_many_isolated, in fact while reviewing this code I was not impressed by too_many_isolated. For vmscan.c if there's an huge nr_active* list and a tiny nr_inactive (like after a truncate of filebacked pages or munmap of anon memory) there's no reason to stall, it's better to go ahead and let it refile more active pages. The too_many_isolated in compaction.c looks a whole lot better than the vmscan.c one as that takes into account the active pages too... But I refrained to make any change in this area as I don't think the bug is in too_many_isolated itself. I noticed the count[] array is unsigned int, but it looks ok (especially for 32bit ;) because the isolation is limited. Both bugs were reported on 32bit x86 UP builds with PREEMPT=y. The stat accounting seem to use atomics on UP so irqs on off or PREEMPT=y/n shouldn't matter if the increment is 1 insn long (plus no irq code should ever mess with nr_isolated)... If it wasn't atomic and irqs or preempt aren't disabled it could be preempt. To avoid confusion: it's not proven that PREEMPT is related, it may be an accident both .config had it on. I'm also unsure why it moves from -1,0,1 I wouldn't expect a single page to be isolated like -1 pages to be isolated, it just looks weird... -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxxx For more info on Linux MM, see: http://www.linux-mm.org/ . Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/ Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>