Question

I want to allocate a large DMA buffer, about 40 MB in size. When I use dma_alloc_coherent(), it fails and what I see is:

------------[ cut here ]------------
WARNING: at mm/page_alloc.c:2106 __alloc_pages_nodemask+0x1dc/0x788()
Modules linked in:
[<8004799c>] (unwind_backtrace+0x0/0xf8) from [<80078ae4>] (warn_slowpath_common+0x4c/0x64)
[<80078ae4>] (warn_slowpath_common+0x4c/0x64) from [<80078b18>] (warn_slowpath_null+0x1c/0x24)
[<80078b18>] (warn_slowpath_null+0x1c/0x24) from [<800dfbd0>] (__alloc_pages_nodemask+0x1dc/0x788)
[<800dfbd0>] (__alloc_pages_nodemask+0x1dc/0x788) from [<8004a880>] (__dma_alloc+0xa4/0x2fc)
[<8004a880>] (__dma_alloc+0xa4/0x2fc) from [<8004b0b4>] (dma_alloc_coherent+0x54/0x60)
[<8004b0b4>] (dma_alloc_coherent+0x54/0x60) from [<803ced70>] (mxc_ipu_ioctl+0x270/0x3ec)
[<803ced70>] (mxc_ipu_ioctl+0x270/0x3ec) from [<80123b78>] (do_vfs_ioctl+0x80/0x54c)
[<80123b78>] (do_vfs_ioctl+0x80/0x54c) from [<8012407c>] (sys_ioctl+0x38/0x5c)
[<8012407c>] (sys_ioctl+0x38/0x5c) from [<80041f80>] (ret_fast_syscall+0x0/0x30)
---[ end trace 4e0c10ffc7ffc0d8 ]---

I’ve tried different values and it looks like dma_alloc_coherent() can’t allocate more than 2^25 bytes (32 MB).

How can such a large DMA buffer can be allocated?

Answer 1

After the system has booted up dma_alloc_coherent() is not necessarily reliable for large allocations. This is simply because non-moveable pages quickly fill up your physical memory making large contiguous ranges rare. This has been a problem for a long time.

Conveniently a recent patch-set may help you out, this is the contiguous memory allocator which appeared in kernel 3.5. If you’re using a kernel with this then you should be able to pass cma=64M on your kernel command line and that much memory will be reserved (only moveable pages will be placed there). When you subsequently ask for your 40M allocation it should reliably succeed. Simples!

For more information check out this LWN article:

https://lwn.net/Articles/486301/