How can I shrink the Linux page cache from within kernel space?

[David Sorber]

I posted this question on stackoverflow yesterday but it occurs to me that
this is a far better place to ask:

I'm working on a system that involves some custom hardware and a custom
Linux device driver I wrote for the hardware. The system occasionally needs
to move large amounts of data very rapidly and therefore my driver
dynamically (i.e. when needed) allocates large (1 GB) DMA buffers which are
used and then freed when they are no longer needed. To allocate such large
buffers I actually allocate a bunch of smaller buffers (256 X 4MB) using
dma_alloc_coherent and then map them contiguously into user space using
remap_pfn_range. This works very well most of the time.

During testing, after the system has been running test cases for a long
time, I sometimes see DMA allocation failures where one of the
dma_alloc_coherent calls in my driver fails which causes my application
layer software to crash. I was finally able to track down this problem and
I discovered that when I see DMA allocation failures the Linux kernel page
cache is very full.

For example, on the last failure that I captured the page cache filled 27
GB of the 32 GB of RAM on my system. I suspected that the page cache
"fullness" was causing dma_alloc_coherent calls to fail. To test this
theory I manually emptied the page cache using:

# echo 1 > /proc/sys/vm/drop_caches

This dropped the size of the cache from 27 GB to 94 MB and I was able to
allocate 20+ 1 GB DMA buffers with no issues.

Clearly the page cache is a beneficial thing so I would prefer not to have
to completely empty it every time I run out of space when allocating DMA
buffers. My questions is this: how can I dynamically shrink the page cache
in kernel space such that if a call to dma_alloc_coherent fails I can
recover just enough space so that I can retry the call and have it succeed?

My system is x86_64 based running a 3.16.x Linux kernel.

I have found some vague references that suggest what I'm attempting may be
possible, for example "These objects are automatically reclaimed by the
kernel when memory is needed elsewhere on the system." (from:
https://www.kernel.org/doc/Documentation/sysctl/vm.txt). But I have not yet
found any specifics that indicate how the memory is reclaimed.

Any assistance with this would be greatly appreciated!

-David
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