Re: [RFC 0/3] Recursive reclaim (on __PF_MEMALLOC)

Christoph Lameter
2007-09-13T18:32:41Z

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.