The Linux memory management subsystem has changed dramatically since the 2.0 kernel came out. Happily, however, the changes to its programming interface have been much smaller and easier to deal with.
kmalloc and kfree have
remained essentially constant between Linux 2.0 and 2.4. Access to
high memory, and thus the
flag, was added starting with kernel 2.3.23;
sysdep.h fills the gaps and allows for 2.4
semantics to be used in 2.2 and 2.0.
The lookaside cache functions were introduced in Linux 2.1.23, and were simply not available in the 2.0 kernel. Code that must be portable back to Linux 2.0 should stick with kmalloc and kfree. Moreover, kmem_destroy_cache was introduced during 2.3 development and has only been backported to 2.2 as of 2.2.18. For this reason scullc refuses to compile with a 2.2 kernel older than that.
__get_free_pages in Linux 2.0 had a
third, integer argument called
dma; it served the
same function that the
serves in modern kernels but it was not merged in the
flags argument. To address the problem,
sysdep.h passes 0 as the third argument to the
2.0 function. If you want to request DMA pages and be backward
compatible with 2.0, you need to call
get_dma_pages instead of using
vmalloc and vfree are
unchanged across all 2.x kernels. However, the
ioremap function was called
vremap in the 2.0 days, and there was no
iounmap. Instead, an I/O mapping obtained with
vremap would be freed with
vfree. Also, the header
<linux/vmalloc.h> didn’t exist in 2.0; the
functions were declared by
instead. As usual,
sysdep.h makes 2.4 code work
with earlier kernels; it also includes
<linux/mm.h> is included, thus hiding this
difference as well.