(3/7/12 12:58 AM), David Rientjes wrote:

On Wed, 7 Mar 2012, KOSAKI Motohiro wrote:

quoted

quoted

It's unnecessary to BUG() in situations when a mempolicy has an
unsupported mode, it just means that a mode doesn't have complete coverage
in all mempolicy functions -- which is an error, but not a fatal error --
or that a bit has flipped.  Regardless, it's sufficient to warn the user
in the kernel log of the situation once and then proceed without crashing
the system.

This patch converts nearly all the BUG()'s in mm/mempolicy.c to
WARN_ON_ONCE(1) and provides the necessary code to return successfully.

I'm sorry. I simple don't understand the purpose of this patch. every
mem policy  syscalls have input check then we can't hit BUG()s in
mempolicy.c. To me, BUG() is obvious notation than WARN_ON_ONCE().

Right, this patch doesn't functionally change anything except it will (1)
continue to warn users when there's a legitimate mempolicy code error by
way of WARN_ON_ONCE() (which is good), just without crashing the machine
unnecessarily and (2) allow the system to stay alive since no mempolicy
error changed by this bug is fatal.  We should only be using BUG() when
the side-effects of continuing are fatal; doing WARN_ON_ONCE(1) is
sufficient annotation, I think, that this code should never be reached --
BUG() has no advantage here.

quoted

We usually use WARN_ON_ONCE() for hw drivers code. Because of, the
warn-on mean "we believe this route never reach, but we afraid there
is crazy buggy hardware".

And, now BUG() has renreachable() annotation. why don't it work?


#define BUG()                                                   \
do {                                                            \
         asm volatile("ud2");                                    \
         unreachable();                                          \
} while (0)

That's not compiled for CONFIG_BUG=n; such a config fallsback to
include/asm-generic/bug.h which just does

	#define BUG()	do {} while (0)

because CONFIG_BUG specifically _wants_ to bypass BUG()s and is reasonably
protected by CONFIG_EXPERT.

So, I strongly suggest to remove CONFIG_BUG=n. It is neglected very long time and
much plenty code assume BUG() is not no-op. I don't think we can fix all place.

Just one instruction don't hurt code size nor performance.

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