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SubjectRE: [Linaro-mm-sig] [PATCHv16 0/9] Contiguous Memory Allocator


On Monday, October 10, 2011 2:08 PM Maxime Coquelin wrote:

> On 10/06/2011 03:54 PM, Marek Szyprowski wrote:
> > Welcome everyone again,
> >
> > Once again I decided to post an updated version of the Contiguous Memory
> > Allocator patches.
> >
> > This version provides mainly a bugfix for a very rare issue that might
> > have changed migration type of the CMA page blocks resulting in dropping
> > CMA features from the affected page block and causing memory allocation
> > to fail. Also the issue reported by Dave Hansen has been fixed.
> >
> > This version also introduces basic support for x86 architecture, what
> > allows wide testing on KVM/QEMU emulators and all common x86 boxes. I
> > hope this will result in wider testing, comments and easier merging to
> > mainline.
> >
> > I've also dropped an examplary patch for s5p-fimc platform device
> > private memory declaration and added the one from real life. CMA device
> > private memory regions are defined for s5p-mfc device to let it allocate
> > buffers from two memory banks.
> >
> > ARM integration code has not been changed since last version, it
> > provides implementation of all the ideas that has been discussed during
> Hello Marek,
> We are currently testing CMA (v16) on Snowball platform.
> This feature is very promising, thanks for pushing it!
> During our stress tests, we encountered some problems :
> 1) Contiguous allocation lockup:
> When system RAM is full of Anon pages, if we try to allocate a
> contiguous buffer greater than the min_free value, we face a
> dma_alloc_from_contiguous lockup.
> The expected result would be dma_alloc_from_contiguous() to fail.
> The problem is reproduced systematically on our side.

Thanks for the report. Do you use Android's lowmemorykiller? I haven't
tested CMA on Android kernel yet. I have no idea how it will interfere
with Android patches.

> 2) Contiguous allocation fail:
> We have developed a small driver and a shell script to
> allocate/release contiguous buffers.
> Sometimes, dma_alloc_from_contiguous() fails to allocate the
> contiguous buffer (about once every 30 runs).
> We have 270MB Memory passed to the kernel in our configuration,
> and the CMA pool is 90MB large.
> In this setup, the overall memory is either free or full of
> reclaimable pages.

Yeah. We also did such stress tests recently and faced this issue. I've
spent some time investigating it but I have no solution yet.

The problem is caused by a page, which is put in the CMA area. This page
is movable, but it's address space provides no 'migratepage' method. In
such case mm subsystem uses fallback_migrate_page() function. Sadly this
function only returns -EAGAIN. The migration loops a few times over it
and fails causing the fail in the allocation procedure.

We are investing now which kernel code created/allocated such problematic
pages and how to add real migration support for them.

> For now, we didn't had time to investigate further theses problems.
> Have you already faced this kind of issues?
> Could someone testing CMA on other boards confirm/infirm theses
> problems?

Best regards
Marek Szyprowski
Samsung Poland R&D Center

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