Re: [PATCH/RFCv4 0/6] The Contiguous Memory Allocator framework

[Andrew Morton]

On Fri, 20 Aug 2010 15:15:10 +0200
Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:

> On Fri, 2010-08-20 at 11:50 +0200, Michal Nazarewicz wrote:
> > Hello everyone,
> > 
> > The following patchset implements a Contiguous Memory Allocator.  For
> > those who have not yet stumbled across CMA an excerpt from
> > documentation:
> > 
> >    The Contiguous Memory Allocator (CMA) is a framework, which allows
> >    setting up a machine-specific configuration for physically-contiguous
> >    memory management. Memory for devices is then allocated according
> >    to that configuration.
> > 
> >    The main role of the framework is not to allocate memory, but to
> >    parse and manage memory configurations, as well as to act as an
> >    in-between between device drivers and pluggable allocators. It is
> >    thus not tied to any memory allocation method or strategy.
> > 
> > For more information please refer to the second patch from the
> > patchset which contains the documentation.
> 
> So the idea is to grab a large chunk of memory at boot time and then
> later allow some device to use it?
> 
> I'd much rather we'd improve the regular page allocator to be smarter
> about this. We recently added a lot of smarts to it like memory
> compaction, which allows large gobs of contiguous memory to be freed for
> things like huge pages.
> 
> If you want guarantees you can free stuff, why not add constraints to
> the page allocation type and only allow MIGRATE_MOVABLE pages inside a
> certain region, those pages are easily freed/moved aside to satisfy
> large contiguous allocations.

That would be good.  Although I expect that the allocation would need
to be 100% rock-solid reliable, otherwise the end user has a
non-functioning device.  Could generic core VM provide the required level
of service?

Anyway, these patches are going to be hard to merge but not impossible.
Keep going.  Part of the problem is cultural, really: the consumers of
this interface are weird dinky little devices which the core MM guys
tend not to work with a lot, and it adds code which they wouldn't use.

I agree that having two "contiguous memory allocators" floating about
on the list is distressing.  Are we really all 100% diligently certain
that there is no commonality here with Zach's work?

I agree that Peter's above suggestion would be the best thing to do. 
Please let's take a look at that without getting into sunk cost
fallacies with existing code!

It would help (a lot) if we could get more attention and buyin and
fedback from the potential clients of this code.  rmk's feedback is
valuable.  Have we heard from the linux-media people?  What other
subsystems might use it?  ieee1394 perhaps?  Please help identify
specific subsystems and I can perhaps help to wake people up.

And I agree that this code (or one of its alternatives!) would benefit
from having a core MM person take a close interest.  Any volunteers?

Please cc me on future emails on this topic?
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