On Thu, Nov 01, 2012 at 04:48:41PM -0700, Hugh Dickins wrote: > On Thu, 1 Nov 2012, Dave Jones wrote: > > On Thu, Nov 01, 2012 at 04:03:40PM -0700, Hugh Dickins wrote: > > > > > > Except... earlier in the thread you explained how you hacked > > > #define VM_BUG_ON(cond) WARN_ON(cond) > > > to get this to come out as a warning instead of a bug, > > > and now it looks as if "a user" has here done the same. > > > > > > Which is very much a user's right, of course; but does > > > make me wonder whether that user might actually be davej ;) > > > > indirectly. I made the same change in the Fedora kernel a while ago > > to test a hypothesis that we weren't getting any VM_BUG_ON reports. > > Fedora turns on CONFIG_DEBUG_VM? Yes. > All mm developers should thank you for the wider testing exposure; > but I'm not so sure that Fedora users should thank you for turning > it on - really it's for mm developers to wrap around !assertions or > more expensive checks (e.g. checking calls) in their development. The last time I did some benchmarking the impact wasn't as ridiculous as say lockdep, or spinlock debug. Maybe the benchmarks I was using weren't pushing the VM very hard, but it seemed to me that the value in getting info in potential problems early was higher than a small performance increase. > Or did I read a few months ago that some change had been made to > such definitions, and VM_BUG_ON(contents) are evaluated even when > the config option is off? I do hope I'm mistaken on that. Pretty sure that isn't the case. I remember Andrew chastising people a few times for putting checks in VM_BUG_ON's that needed to stay around even when the config option was off. Perhaps you were thinking of one of those incidents ? Dave -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>