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[ogfs-dev]vmalloc/vfree bugs in osi_malloc/osi_free



I just found a paper "How NOT to write kernel drivers".  In
section 2.2, some ogfs code is used as a bad example:

--
  Most tutorials warn about using kmalloc for al-locating big areas of
  memory, and it seems a lot of people also notice these limits in
  practice:

    static __inline void *
    osi_malloc(unsigned int size)
    {
        if (size > 2*PAGE_SIZE)
            return __vmalloc(size, GFP_NOFS|__GFP_HIGHMEM, PAGE_KERNEL);
        return kmalloc(size, GFP_NOFS);
    } 
[due to cut and paste problems with acroread, I inserted the code
from the ogfs cvs]

  from OpenGFS 0.99.2 There are several problems with such an approach.
  The first one is performance: vmalloc() ed memory requires more TLBs,
  which are a rare resource on most CPUs. A more serious problem arises
  when the osi_free() routine gets involved: it calls vfree() for the
  vmalloc()ed memory, and it s illegal to call vfree() from inter-rupt
  context. This abstraction makes it okay to call the osi_free()
  function from interrupt context sometimes, but not for big
  allocations. A third issue that most tutorials don t mention is that
  while you can use vmalloc() to allo-cate larger chunks of memory, the
  total amount you can allocate is rather limited, in the or-der of 64
  Megabytes on modern machines (de-pending on what PCI hardware is
  present).
--


And indeed this code ise still in the sources
(include/osi_linux_2_4.h).  I'm not deep enough into writing
kernel code to figure out what the correct fix is.  Does the
author suggest to always use kmalloc instead of vmalloc?

Ciao

Dominik ^_^  ^_^


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