mmap()

[Kamal R. Prasad]

John-Mark Gurney wrote:

>Kamal R. Prasad wrote this message on Sun, Dec 05, 2004 at 00:38 +0530:
>  
>
>>I wrote an mmap() interface for a USB device. But when I made a call to 
>>it using mmap(), I saw that mmap interface is called 3-4 times. The 
>>calls are being made from within mmap() i.e. sys/vm/vm_mmap.c. Can 
>>someone tell me if there is something like a re-try going on for some 
>>reason?
>>From userspace, I called it as 
>>addr = mmap(NULL, 1024, PROT_READ, MAP_SHARED, fd, 0);
>>The version of OS is Freebsd 5.3(stable).
>>    
>>
>
>each mmap call will be called whenever the kernel needs to map that
>specific page..  say you map 16k of your device...  it won't map any
>of the 16k until the first time it is accessed, then it will call the
>mmap routine for each page as it is accessed...  it is also possible that
>memory for the mappings needs to be reclaimed, at which point those page
>mappings will be reaped, and your mmap function will be called again when
>they need to be accessed again..
>
>  
>
Thanks. But then, the mmap'ed() address was yet to be used by the user 
space and I don't see the need for multiple calls to my interface almost 
as if in a while loop.  Is there any return value from the mmap() 
interface that could trigger another call? I am returning 0 after 
setting the param to vtophys(kernel virtual address).

regards
-kamal