On Thu 2017-12-07 15:02:21, Michal Hocko wrote: > On Thu 07-12-17 13:58:05, Cyril Hrubis wrote: > > Hi! > > > >> (It does seem unfortunate that the man page cannot help the programmer > > > >> actually write correct code here. He or she is forced to read the kernel > > > >> implementation, in order to figure out the true alignment rules. I was > > > >> hoping we could avoid that.) > > > > > > > > It would be nice if we had this information exported somehere so that we > > > > do not have to rely on per-architecture ifdefs. > > > > > > > > What about adding MapAligment or something similar to the /proc/meminfo? > > > > > > > > > > What's the use case you envision for that? I don't see how that would be > > > better than using SHMLBA, which is available at compiler time. Because > > > unless someone expects to be able to run an app that was compiled for > > > Arch X, on Arch Y (surely that's not requirement here?), I don't see how > > > the run-time check is any better. > > > > I guess that some kind of compile time constant in uapi headers will do > > as well, I'm really open to any solution that would expose this constant > > as some kind of official API. > > I am not sure this is really feasible. It is not only a simple alignment > thing. Look at ppc for example (slice_get_unmapped_area). Other > architectures might have even more complicated rules e.g. arm and its > cache_is_vipt_aliasing. Also this applies only on MAP_SHARED || file > backed mappings. > > I would really leave dogs sleeping... Trying to document all this in the > man page has chances to confuse more people than it has chances to help > those who already know all these nasty details. You don't have to provide all the details, but warning that there's arch- specific magic would be nice... Pavel (english) http://www.livejournal.com/~pavelmachek (cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
