Hi Andrew,
Thank you for your comments and interest in this!
On 08/26/2010 07:58 AM, Andrew Morton wrote:
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.
This is encouraging, thanks. Merging a contiguous allocator seems like a
lost cause, with a relative disinterest of non-embedded people, and on
the other hand because of the difficulty to satisfy those actually
interested. With virtually everybody having their own, custom solutions,
agreeing on one is nearly impossible.
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 think Zach's work is more focused on IOMMU and on unifying virtual
memory handling. As far as I understand, any physical allocator can be
plugged into it, including CMA. CMA solves a different set of problems.
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.
As a media developer myself, I talked with people and many have
expressed their interest. Among them were developers from ST-Ericsson,
Intel and TI, to name a few. Their SoCs, like ours at Samsung, require
contiguous memory allocation schemes as well.
I am working on a driver framework for media for memory management (on
the logical, not physical level). One of the goals is to allow plugging
in custom allocators and memory handling functions (cache management,
etc.). CMA is intended to be used as one of the pluggable allocators for
it. Right now, many media drivers have to provide their own, more or
less complicated, memory handling, which is of course undesirable. Some
of those make it to the kernel, many are maintained outside the mainline.
The problem is that, as far as I am aware, there have already been quite
a few proposals for such allocators and none made it to the mainline. So
companies develop their own solutions and maintain them outside the
mainline.
I think that the interest is definitely there, but people have their
deadlines and assume that it is close to impossible to have a contiguous
allocator merged.
Your help and support would be very much appreciated. Working in
embedded Linux for some time now, I feel that the need is definitely
there and is quite substantial.
--
Best regards,
Pawel Osciak
Linux Platform Group
Samsung Poland R&D Center
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