kmalloc and vmalloc : Linux kernel memory allocation API Limits

The Intent

To determine how large a memory allocation can be made from within the kernel, via the “usual suspects” – the kmalloc and vmalloc kernel memory allocation APIs, in a single call.

Lets answer this question using two approaches: one, reading the source, and two, trying it out empirically on the system.
(Kernel source from kernel ver 3.0.2; tried out on kernel ver 2.6.35 on an x86 PC and on the (ARM) BeagleBoard).

Quick Summary

For the impatient:

The upper limit (number of bytes that can be allocated in a single kmalloc request), is a function of:

  • the processor – really, the page size – and
  • the number of buddy system freelists (MAX_ORDER).

On both x86 and ARM, with a standard page size of 4 Kb and MAX_ORDER of 11, the kmalloc upper limit is 4 MB!

The vmalloc upper limit is, in theory, the amount of physical RAM on the system.
In practice, the kernel allocates an architecture (cpu) specific “range” of virtual memory for the purpose of vmalloc: from VMALLOC_START to VMALLOC_END.

In practice, it’s usually a lot less. A useful comment by ugoren points out that:
” in 32bit systems, vmalloc is severely limited by its virtual memory area. For a 32bit x86 machine, with 1GB RAM or more, vmalloc is limited to 128MB (for all allocations together, not just for one).

[EDIT/UPDATE #3 : July ’17]
I wrote a simple kernel module (can download the source code, see the link at the end of this article), to test the kmalloc/vmalloc upper limits; the results are what we expect:
for kmalloc, 4 MB is the upper limit with a single call; for vmalloc, it depends on the vmalloc range.

Also, please realize, the actual amount you can acquire at runtime depends on the amount of physically contiguous RAM available at that moment in time; this can and does vary widely.

Finally, what if one require more than 4 MB of physically contiguous memory? That’s pretty much exactly the reason behind CMA – the Contiguous Memory Allocator! Details on CMA and using it are in this excellent LWN article here. Note that CMA was integrated into mainline Linux in v3.17 (05 Oct 2014). Also, the recommended API interface to use CMA is the ‘usual’ DMA [de]alloc APIs (kernel documentation here and here); don’t try and use them directly.

I kmalloc Limit Tests

First, lets check out the limits for kmalloc :

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