Home ›

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.

______________________

Comments

Comment viewing options

Select your preferred way to display the comments and click "Save settings" to activate your changes.

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

White Paper
More Resources
Fabric-Based Computing Enables Optimized Hyperscale Data Centers

Today’s modular x86 servers are compute-centric, designed as a least common denominator to support a wide range of IT workloads. Those generic, virtualized IT workloads have much different resource optimization requirements than hyperscale and cloud applications. They have resulted in a “one size fits all” enterprise IT architecture that is not optimized for a specific set of IT workloads, and especially not emerging hyperscale workloads, such as web applications, big data, and object storage. In this report, you will learn how shifting the focus from traditional compute-centric IT architectures to an innovative disaggregated fabric-based architecture can optimize and scale your data center.

Learn More

Sponsored by AMD

White Paper
More Resources
Red Hat White Paper: Using an Open Source Framework to Catch the Bad Guy

Built-in forensics, incident response, and security with Red Hat Enterprise Linux 6

Every security policy provides guidance and requirements for ensuring adequate protection of information and data, as well as high-level technical and administrative security requirements for a system in a given environment. Traditionally, providing security for a system focuses on the confidentiality of the information on it. However, protecting the data integrity and system and data availability is just as important. For example, when processing United States intelligence information, there are three attributes that require protection: confidentiality, integrity, and availability.

Learn more about catching the bad guy in this free white paper.

Learn More

Sponsored by DLT Solutions