contigmalloc() and mmap()

[Mark Tinguely]

>  I browsed kernel tree, I found those drivers which use contigmalloc() and
>  contigfree() always call these two kernel interfaces in _attach() and
>  _detach(), but in my driver, I call contigmalloc() in ioctl(), and call
>  contigfree() in a callout function which is set by callout_reset().
>  What I want to know is if contigmalloc() and contigfree() can only be used
>  under some conditions?

Allocating early prevents unfullfilled requests due to fragmentation of
the physical memory. I believe starting in FreeBSD 5.x, contigmalloc()
started to fill physical memory requests from the higher memory locations
to help with fragmentation. FreeBSD 5/6 has a Unified Meory Allocator
that helps allocate/free cycles.

>  And recently, I modified my driver, I allocated a big chunk of contiguous
>  physical memory using contigmalloc() in the driver's _attach() function,
>  and I use a simple first-fit algorithm to manage this memory myself, which
>  mean in ioctl() I use my allocate/deallocate functions instead of 
>  contigmalloc(),
>  in _detach() function contigfree() is called to free the big chunk of 
>  memory,
>  no panic again, but sometimes, process cannot get the correct data from
>  the mmaped memory. I don't know why.

Are you sure that you are allocating page length request on page boundaries?
Are you checking the success/failure of the allocation? Your page faults
before implementing your own allocation sounds like you did not check
the return status from contigmalloc() and were dereferencing a pointer
on page 0.

--Mark Tinguely.