On 08:33 Fri 17 May , Dave Chinner wrote: > On Thu, May 16, 2013 at 02:27:52PM +0200, Oskar Andero wrote: > > On 13:52 Thu 16 May , Dave Chinner wrote: > > > On Thu, May 16, 2013 at 10:42:16AM +0200, Oskar Andero wrote: > > > > The shrinkers must return -1 to indicate that it is busy. Instead, treat > > > > any negative value as busy. > > > > > > Why? The API defines return condition for aborting a scan and gives > > > a specific value for doing that. i.e. explain why should change the > > > API to over-specify the 'abort scan" return value like this. > > > > As I pointed out earlier, looking in to the code (from master): > > if (shrink_ret == -1) > > break; > > if (shrink_ret < nr_before) > > ret += nr_before - shrink_ret; > > > > This piece of code lacks a sanity check and will only function if shrink_ret > > is either greater than zero or exactly -1. If shrink_ret is e.g. -2 this will > > lead to undefined behaviour. > > If a shrinker is returning -2 then the shrinker is broken and needs > fixing. The point is: returning -2 is just as magical and meaningful as returning -1. Usually, returning a negative means "failure" (Chapter 16 CodingStyle), not a perfectly valid "abort scan" as in this piece of code. > > > FWIW, using "any" negative number for "abort scan" is a bad API > > > design decision. It means that in future we can't introduce > > > different negative return values in the API if we have a new to. > > > i.e. each specific negative return value needs to have the potential > > > for defining a different behaviour. > > > > An alternative to my patch would be to add: > > if (shrink_ret < -1) > > /* handle illegal return code in some way */ > > How? We have one valid negative return code. WTF are we supposed to > do if a shrinker is passing undefined return values? IOWs, the only > sane thing to do is: > > BUG_ON(shrink_ret < -1); Yes, of course! BUG_ON() is the proper way to handle an illegal value. Now we are getting somewhere! > > > So if any change needs to be made, it is to change the -1 return > > > value to an enum and have the shrinkers return that enum when they > > > want an abort. > > > > I am all for an enum, but I still believe we should handle the case where > > the shrinkers return something wicked. > > Which bit of "broken shrinkers need to be fixed" don't you > understand? A BUG_ON() will make sure they get fixed - anything else > that allows broken shrinkers to continue functioning is a completely > unacceptible solution. BUG_ON() is perfect IMO and if everyone is ok with that I will send version 2 of my patch. Now there is just the matter of returning hardcoded -1. Would an enum in shrinker.h add any value? I have gotten different feedback on this - some say yea, others nay. I think I have motivated it enough in this thread, so I am not going to repeat myself. -Oskar -- 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>