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

Question

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!

Answer 1

TL;DR : Scan for active superblocks and drop references to non-dirty ones until you have reclaimed as much system memory as you need. (or you finally run out of references to active superblocks.)

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How to write kernel code to dynamically shrink the fs page-cache,
to recover just enough space so that a subsequent call to dma_alloc_coherent() succeeds?

To answer this question, let us take a look at what the "drop_caches operation" did to reduce the fs page-cache from 27GB to 94MB on your system.

1. echo 1 > /proc/sys/vm/drop_caches
   invokes
   drop_caches_sysctl_handler()

2. which in turn invokes iterate_supers() and
   passes it the pointer to the function drop_pagecache_sb().

What happens next is that iterate_supers() scans for active superblocks and everytime it finds one, it calls drop_pagecache_sb(), passing it a reference to the active superblock.

This iterative procedure continues until references to all the active superblocks are freed from the fs page-cache. This is a non-destructive operation and will only free blocks that are completely unused. Dirty-objects will continue to be in use until written out to disk and are not free-able. If you run sync first to flush them out to disk, the "drop_caches operation" tends to free more memory.

Since you are interested in running this process to reclaim a limited/known amount of memory i.e. what is soon going to be requested using dma_alloc_coherent(), you simply need to implement the above functionality with an additional check at the end of each iteration and abort the superblock scan immediately once the amount of free system memory crosses the desired level.

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A couple of points to keep in mind to further optimise this procedure :

• Is there a preference for certain block devices over others?
  You may want to iterate over active superblocks of the block devices that you do not care about first. If enough memory is not reclaimed, then scan the block devices that you would prefer to retain in the fs page-cache unless absolutely necessary to reclaim required memory. get_active_super() might be of help here.

• iterate_supers_type() seems interesting
  It allows one to iterate over superblocks of specific file_system_type

_{Please note that this is a speculative solution based purely on the analysis of existing code within the Linux kernel that you have observed to already solve your problem. Once the above approach is implemented, it will only allow you to control the same i.e. attempt to reclaim fs page-cache memory only to the extent required for your immediate needs.}