linux-kernel - Re: [RFC 0/3] Recursive reclaim (on __PF

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Message-ID: <Pine.LNX.4.64.0709131128050.9546@schroedinger.engr.sgi.com>
Date:	Thu, 13 Sep 2007 11:32:26 -0700 (PDT)
From:	Christoph Lameter <clameter@....com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
cc:	Nick Piggin <npiggin@...e.de>,
	Daniel Phillips <phillips@...nq.net>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, akpm@...ux-foundation.org,
	dkegel@...gle.com, David Miller <davem@...emloft.net>
Subject: Re: [RFC 0/3] Recursive reclaim (on __PF_MEMALLOC)

On Thu, 13 Sep 2007, Peter Zijlstra wrote:

> 
> > > Every user of memory relies on the VM, and we only get into trouble if
> > > the VM in turn relies on one of these users. Traditionally that has only
> > > been the block layer, and we special cased that using mempools and
> > > PF_MEMALLOC.
> > > 
> > > Why do you object to me doing a similar thing for networking?
> > 
> > I have not seen you using mempools for the networking layer. I would not 
> > object to such a solution. It already exists for other subsystems.
> 
> Dude, listen, how often do I have to say this: I cannot use mempools for
> the network subsystem because its build on kmalloc! What I've done is
> build a replacement for mempools - a reserve system - that does work
> similar to mempools but also provides the flexibility of kmalloc.
> 
> That is all, no more, no less.

Its different since it becomes a privileged player that can suck all 
the available memory out of the page allocator.

> I'm confused by this, I've never claimed part of, or such a thing. All
> I'm saying is that because of the circular dependency between the VM and
> the IO subsystem used for swap (not file backed paging [*], just swap)
> you have to do something special to avoid deadlocks.

How are dirty file backed pages different? They may also be written out 
by the VM during reclaim.

> > Replacing the mempools for the block layer sounds pretty good. But how do 
> > these various subsystems that may live in different portions of the system 
> > for various devices avoid global serialization and livelock through your 
> > system? 
> 
> The reserves are spread over all kernel mapped zones, the slab allocator
> is still per cpu, the page allocator tries to get pages from the nearest
> node.

But it seems that you have unbounded allocations with PF_MEMALLOC now for 
the networking case? So networking can exhaust all reserves?

> > And how is fairness addresses? I may want to run a fileserver on 
> > some nodes and a HPC application that relies on a fiberchannel connection 
> > on other nodes. How do we guarantee that the HPC application is not 
> > impacted if the network services of the fileserver flood the system with 
> > messages and exhaust memory?
> 
> The network system reserves A pages, the block layer reserves B pages,
> once they start getting pages from the reserves they go bean counting,
> once they reach their respective limit they stop.

That sounds good.
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