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Date: Tue, 21 Feb 2012 14:12:58 -0800 (PST)
From: Hugh Dickins <hughd@google.com>
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To: Konstantin Khlebnikov <khlebnikov@openvz.org>
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Subject: Re: [PATCH 9/10] mm/memcg: move lru_lock into lruvec
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On Wed, 22 Feb 2012, Konstantin Khlebnikov wrote:
> Hugh Dickins wrote:
> > 
> > I'll have to come back to think about your locking later too;
> > or maybe that's exactly where I need to look, when investigating
> > the mm_inline.h:41 BUG.
> 
> pages_count[] updates looks correct.
> This really may be bug in locking, and this VM_BUG_ON catch it before
> list-debug.

I've still not got into looking at it yet.

You're right to mention DEBUG_LIST: I have that on some of the machines,
and I would expect that to be the first to catch a mislocking issue.

In the past my problems with that BUG (well, the spur to introduce it)
came from hugepages.

> > 
> > But at first sight, I have to say I'm very suspicious: I've never found
> > PageLRU a good enough test for whether we need such a lock, because of
> > races with those pages on percpu lruvec about to be put on the lru.
> > 
> > But maybe once I look closer, I'll find that's handled by your changes
> > away from lruvec; though I'd have thought the same issue exists,
> > independent of whether the pending pages are in vector or list.
> 
> Are you talking about my per-cpu page-lists for lru-adding?

Yes.

> This is just an unnecessary patch, I don't know why I include it into v2 set.
> It does not protect anything.

Okay.

Hugh