mmaping /dev/mem

[sridhar vaidyanathan]

I have a problem mmaping /dev/mem on some address in RAM. I am aware of 
caveats,but I am trying to mmap a region which I am sure is not in use by the 
kernel(some additional code does this and returns a physical address which is 
what I use for mmap). The mmap call itself succeeds and /proc/pid/maps also 
shows that region, but I am unable to see what I write in target memory.I also 
tried with the O_SYNC flag as I was wondering is caching had anything to do 
with the results that I was seeing.No effect.
This however works with a mem= option and when the mmap region falls out of the 
mem= boundary.
any clues?
Please cc as I am not subscribed
sridhar