linux-kernel - Re: [PATCH] Memory management livelock

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Date:	Fri, 3 Oct 2008 07:43:46 -0400 (EDT)
From:	Mikulas Patocka <mpatocka@...hat.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
cc:	Nick Piggin <nickpiggin@...oo.com.au>,
	linux-kernel@...r.kernel.org, agk@...hat.com, mbroz@...hat.com,
	chris@...chsys.com
Subject: Re: [PATCH] Memory management livelock

On Thu, 2 Oct 2008, Andrew Morton wrote:

> On Fri, 3 Oct 2008 13:47:21 +1000 Nick Piggin <nickpiggin@...oo.com.au> wrote:
> 
> > > I expect there's no solution which avoids blocking the writers at some
> > > stage.
> > 
> > See my other email. Something roughly like this would do the trick
> > (hey, it actually boots and runs and does fix the problem too).
> 
> It needs exclusion to protect all those temp tags.  Is do_fsync()'s
> i_mutex sufficient?  It's qute unobvious (and unmaintainable?) that all
> the callers of this stuff are running under that lock.

That filemap_fdatawrite and filemap_fdatawait in fsync() aren't really 
called under i_mutex (see do_fsync).

So the possible solutions are:

1. Add jiffies when the page was diried and wroteback to struct page
+ no impact on locking and concurrency
- increases the structure by 8 bytes

2. Stop the writers when the starvation happens (what I did)
+ doesn't do any locking if the livelock doesn't happen
- locks writers when the livelock happens (I think it's not really serious 
--- because very few people complained about the livelock, very few people 
will see performance degradation from blocking the writers).

3. Add another bit to radix tree (what Nick did)
+ doesn't ever block writers
- unconditionally takes the lock on fsync path and serializates concurrent 
syncs/fsyncs. Probably low overhead too ... or I don't know, is there any 
possible situation when more processes execute sync() in parallel and user 
would see degradations if those syncs were serialized?

Mikulas
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