Re: [PATCH] mm: compaction: Abort compaction if too many pages areisolated and caller is asynchronous

[Mel Gorman]

On Mon, May 30, 2011 at 07:53:34PM +0200, Andrea Arcangeli wrote:
> 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.
> 

Yep, !CONFIG_SMP is important.

> > 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.

I'm no longer considering it if CONFIG_SMP is not set. 

> 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.
> 

I didn't pay as close attention as I should either. I was out for a few
days when that thread happened and it had gone quiet by the time I came
back. I didn't check if it ever got resolved.

> 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?

No. Now that I look at the config, CONFIG_SMP was not set.

> I'd prefer to clear out
> this error before improving the too_many_isolated,

Agreed. So far, it has been a impossible to reproduce but it's
happened to enough people that it must exist. I have a similar
config now and have applied a debugging patch to warn when the count
gets screwed up. It hasn't triggered yet so it must be due to some
difficult-to-hit-race.

> 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 rotating of lists should already have happened but you could be
right if a process was stalled in too_many_isolated for a long period of
time that the lists would become imbalanced. It'd be saved by kswapd
running at the same time and rotating the list but it might need to be
revisisted at some point. It's unrelated to the current problem though.

> 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.
> 

Not a bug that would lock up the machine at least.

> I noticed the count[] array is unsigned int, but it looks ok
> (especially for 32bit ;) because the isolation is limited.
> 

Agreed, this is not a wrapping problem.

> 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...
> 

Hmm, looking at the zoneinfo, we can probably rule out some race
related to page flags. If we someone isolated a page as anon and put
it back as file, we'd decrement nr_isolated_file inappropriate to -1
but nr_isolated_anon would be stuck on 1 to match it.

I'm looking at __collapse_huge_page_isolate and it always assumes
it isolated anonymous pages. mmap_sem should be held as well as the
pagetable lock and we hold the page lock while isolating from the LRU
which seems fine but I wonder could there be another condition that
allows a !PageSwapBacked page to sneak in there?

-- 
Mel Gorman
SUSE Labs
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