Kernel Korner - Allocating Memory in the Kernel
With a little understanding, getting a hold of memory in the kernel is demystified and not too much more difficult to do than it is in user space. A few simple rules of thumb can go a long way:
Decide whether you can sleep (that is, whether the call to kmalloc() can block). If you are in an interrupt handler, in a bottom half, or if you hold a lock, you cannot. If you are in process context and do not hold a lock, you probably can.
If you can sleep, specify GFP_KERNEL.
If you cannot sleep, specify GFP_ATOMIC.
If you need DMA-capable memory (for example, for an ISA or broken PCI device), specify GFP_DMA.
Always check for and handle a NULL return value from kmalloc().
Do not leak memory; make sure you call kfree() somewhere.
Ensure that you do not race and call kfree() multiple times and that you never access a block of memory after you free it.
For more information, check out these files in your kernel source tree.
include/linux/gfp.h: home of the allocation flags.
include/linux/slab.h: definitions of kmalloc(), et al.
mm/page_alloc.c: page allocation functions.
mm/slab.c: implementation of kmalloc(), et al.
Robert Love (firstname.lastname@example.org) is a kernel hacker at MontaVista Software and a student at the University of Florida. He is the author of Linux Kernel Development. Robert enjoys fine wine and lives in Gainesville, Florida.
Free DevOps eBooks, Videos, and more!
Regardless of where you are in your DevOps process, Linux Journal can help!
We offer here the DEFINITIVE DevOps for Dummies, a mobile Application Development Primer, and advice & help from the expert sources like:
- Linux Journal