I am trying to get DMAable memory in user space by
allocating it in kernel space and mmaping it to the
user space application. The requirement is that the
application will keep on doing allocations for this
memory and keep freeing it and it may not be for full
size pages(eg a few bytes only). For this I have
written a prototype driver and user application which
I have attached with the mail. Basically, the steps I
am following are:

1. implement the mmap() function in the driver
2. in user space the dma_malloc() call which is the
wrapper function to get dmaable memory will do the
following:
     a. call the mmap() on the driver where driver
allocates the private house keeping data structure in
vma(vma->vm_private_data) and initializes vmops, so
that munmap() will call the close() implemented by
driver
     b. does an ioctl with the start of vma address
which it gets from mmap() call above and the
allocation size. driver allocates the memory and calls
remap_page_range() to map the allocated region
directly in the user space and calls mem_map_reserve()
to reserve this area before remap. it stores the
pointer to the
kmalloc'ed area in vma->vm_private_data so that it
could be freed during close() of vma. Ioctl returns
the offset into page for the allocated memory
to the user space.
     c. the wrapper function returns the pointer of
the kmalloced area by adding this offset to the page
address returned by mmap().

This works fine for the first allocation in any page
and the corresponding free. But it causes an oops if
allocations are done more than once.

Can anybody shed some light on what I am doing wrong
which is causing this oops(divide error: 0000).

What I have observed is that if I unmap the page and
then free the kernel pointer(through the ioctl) this
oops does not happen. Can someone please tell me why
this behaviour, or am I mising something at the basic
level itself.

Thanks in advance for any information.

Regards. 




	
		
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