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Kernel Korner - Allocating Memory in the Kernel

Issue 116

From Issue #116
December 2003

Dec 01, 2003  By Robert Love
 in
In this article, Robert offers a refresher on kernel memory allocation and how it has changed for the 2.6 kernel.
Conclusion

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.

Resources

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 (rml@tech9.net) 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.

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Kernel flags

Anonymous's picture
Submitted by Anonymous (not verified) on Mon, 07/19/2010 - 22:29.

Hi, i see in my aircraft linux system flag 7 and flag 0 from 2 different kernel versions. What are they? Pls help. Thks

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