On 08/26/2010 08:31 AM, Jonathan Corbet wrote:
On Wed, 25 Aug 2010 15:58:14 -0700
Andrew Morton<akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
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?
The original OLPC has a camera controller which requires three contiguous,
image-sized buffers in memory. That system is a little memory constrained
(OK, it's desperately short of memory), so, in the past, the chances of
being able to allocate those buffers anytime some kid decides to start
taking pictures was poor. Thus, cafe_ccic.c has an option to snag the
memory at initialization time and never let go even if you threaten its
family. Hell hath no fury like a little kid whose new toy^W educational
tool stops taking pictures.
That, of course, is not a hugely efficient use of memory on a
memory-constrained system. If the VM could reliably satisfy those
allocation requestss, life would be wonderful. Seems difficult. But it
would be a nicer solution than CMA, which, to a great extent, is really
just a standardized mechanism for grabbing memory and never letting go.
The main problem is of course fragmentation, for this there is no
solution in CMA. It has a feature intended to at least reduce memory
usage though, if only a little bit. It is region sharing. It allows
platform architects to define regions shared by more than one driver, as
explained by Michal in the RFC. So we can at least try to reuse each
chunk of memory as much as possible and not hold separate regions for
each driver when they are not intended to work simultaneously. Not a
silver bullet, but is there any though?
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.
If this code had been present when I did the Cafe driver, I would have used
it. I think it could be made useful to a number of low-end camera drivers
if the videobuf layer were made to talk to it in a way which Just Works.
I am working on new videobuf which will (hopefully) Just Work. CMA is
intended to be pluggable into it, as should be any other allocator for
that matter.
--
Best regards,
Pawel Osciak
Linux Platform Group
Samsung Poland R&D Center
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