On Fri, 01 Mar 2013 16:48:12 +0530, sandeep kumar said: > Don't you think it should throw panic()while calling the ioremap() itself. > Because this sounds like a serious violation... As you noted, it does give you a warning. That's a kernel design philosophy - to reserve the panic() and BUG() calls for cases where it is *known* that proceeding further is unsafe or impossible. So the kernel does a panic() if it can't start /sbin/init at system boot-up - because without that, further progress is impossible. But once the system is up, we don't panic if PID 1 goes away - because it's possible that the user has an open window, and can su and at least do an orderly shutdown. Similarly, if a device driver gets confused, the driver code may do a BUG_ON() and end up locking up that device because to do anything else may scramble the disk further. But we don't panic() because that will basically wedge the system - and the user loses any chance at dumping the dmesg buffer for debugging or other attempts at an orderly shutdown (in particular, panic() won't sync the filesystems. So even though a BUG() often kills a thread while it holds an important lock, which often leads to the system eventually deadlocking one process at a time, it's still a net win if it doesn't panic but lets the user at least try to run sync. And even BUG_ON() is frowned upon if further progress in a degraded mode is possible (for instance, a networking error that totally locks up one TCP connection, but other connections are still working) - at that point, warn() is the correct thing to do. As in this case - it *is* a serious violation, but the kernel (a) can at least possibly keep going and (b) it's at least possible that the user can recover from it. There's a *very* good chance that if the kernel just does a warn(), the user will say "*facepalm* Stupid typo in the address", fix the typo, and re-try with the correct address. So that's the design philosophy of why it gives you a warning rather than a panic.
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