dma_alloc_coherent fragmentation

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Looping in linux-arm ML:

Discussion Ref: [1](linux-omap mailing list)
While working with Linux OMAP kernel[2] we found that on allocating 4 meg chunks using dma_alloc_coherent and de-allocating with dma_free_coherent in a loop using a test driver[7], the memory is getting fragmented as shown by Ameya's observation [3] finally resulting in dump_stack due to lack of pages.

Searching through previous archives, found [4] which seem to imply that allocating chunks greater than 1 page could cause fragmentation.

I wonder why we see this even if we allocate and free the memory in sequence as in [5]. This seems to happen on 2.6.28 and 2.6.29 but not on the 2.6.27 based kernel[6].

Regards,
Nishanth Menon

Ref:
[1] http://marc.info/?t=123719779100002&r=1&w=2
[2] http://git.kernel.org/?p=linux/kernel/git/tmlind/linux-omap-2.6.git
[3] http://marc.info/?l=linux-omap&m=123722460022336&w=2
[4] http://lkml.indiana.edu/hypermail/linux/kernel/0706.0/0370.html 
[5] http://marc.info/?l=linux-omap&m=123719772811746&w=2
[6] http://git.omapzoom.com/?p=repo/omapkernel.git;
[7] http://marc.info/?l=linux-omap&m=123719772811746&q=p3 
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