locking user space memory in kernel

[Eli Cohen]

Hi,
I need to be able to lock memory allocated in user space and passed to 
my driver, in order to pass it to a dma controller that can maintain a 
translation table for each process. The obvious thing is to use 
sys_mlock() (and sys_munlock() for unlocking) but this function is not 
exported anymore, nore is sys_call_table.  I considered marking the 
relevant vma->vm_flags with VM_LOCKED and calling get_user_pages but 
that could be overkill if I want to lock just a portion of the VMA. 
Currently I do some hacking to find the addresses of sys_mlock/sys_munlock.
I also need to maintain a reference count on the locking /unlocking such 
that a region that has been locked twice will really be unlocked after 
unlocking twice. This needs to support partly overlapping regions. To 
cope with this I have implemented some code on top of calls to 
sys_mlock/sys_munlock to provide this functionality.
Are there more standard ways to get this functionality from the kernel? 
Any help is appreciated.

Thanks
Eli
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