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

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Message-ID: <Pine.LNX.4.64.0809241439580.17357@hs20-bc2-1.build.redhat.com>
Date:	Wed, 24 Sep 2008 14:50:23 -0400 (EDT)
From:	Mikulas Patocka <mpatocka@...hat.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
cc:	linux-kernel@...r.kernel.org, linux-mm@...r.kernel.org,
	agk@...hat.com, mbroz@...hat.com, chris@...chsys.com
Subject: Re: [PATCH] Memory management livelock

> > > yup, that's pretty much unfixable, really, unless new locks are added
> > > which block threads which are writing to unrelated sections of the
> > > file, and that could hurt some workloads quite a lot, I expect.
> > 
> > It is fixable with the patch I sent --- it doesn't take any locks unless 
> > the starvation happens. Then, you don't have to use .nr_to_write for 
> > fsync anymore.
> 
> I agree that the patch is low-impact and relatively straightforward. 
> The main problem is making the address_space larger - there can (and
> often are) millions and millions of these things in memory.  Making it
> larger is a big deal.  We should work hard to seek an alternative and
> afacit that isn't happening here.
> 
> We already have existing code and design which attempts to avoid
> livelock without adding stuff to the address_space.  Can it be modified
> so as to patch up this quite obscure and rarely-occuring problem?

I reworked my patch to use a bit in address_space->flags and hashes wait 
queues, so it doesn't take any extra memory. I'm sending it in three 
parts.
1 - make generic function wait_action_schedule
2 - fix the livelock, the logic is exactly the same as in my previous 
patch, wait_on_bit_lock is used instead of mutexes
3 - remove that nr_pages * 2 limit, because it causes misbehavior and 
possible data loss.

Mikulas
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