On Mon 04-12-17 18:52:27, John Hubbard wrote: > On 12/04/2017 03:31 AM, Mike Rapoport wrote: > > On Sun, Dec 03, 2017 at 06:14:11PM -0800, john.hubbard@xxxxxxxxx wrote: > >> From: John Hubbard <jhubbard@xxxxxxxxxx> > >> > [...] > >> +.IP > >> +Given the above limitations, one of the very few ways to use this option > >> +safely is: mmap() a region, without specifying MAP_FIXED. Then, within that > >> +region, call mmap(MAP_FIXED) to suballocate regions. This avoids both the > >> +portability problem (because the first mmap call lets the kernel pick the > >> +address), and the address space corruption problem (because the region being > >> +overwritten is already owned by the calling thread). > > > > Maybe "address space corruption problem caused by implicit calls to mmap"? > > The region allocated with the first mmap is not exactly owned by the > > thread and a multi-thread application can still corrupt its memory if > > different threads use mmap(MAP_FIXED) for overlapping regions. > > > > My 2 cents. > > > > Hi Mike, > > Yes, thanks for picking through this, and I agree that the above is misleading. > It should definitely not use the word "owned" at all. Re-doing the whole > paragraph in order to make it all fit together nicely, I get this: > > "Given the above limitations, one of the very few ways to use this option > safely is: mmap() an enclosing region, without specifying MAP_FIXED. > Then, within that region, call mmap(MAP_FIXED) to suballocate regions > within the enclosing region. This avoids both the portability problem > (because the first mmap call lets the kernel pick the address), and the > address space corruption problem (because implicit calls to mmap will > not affect the already-mapped enclosing region)." > > ...how's that sound to you? I'll post a v3 soon with this. It sounds to me you are trying to tell way to much while actually being a bit misleading. Even sub-range MAP_FIXED is not multi-thread safe. Really the more corner cases you will try to cover the worse the end result will end up. I would just try to be simple here and mention the address space corruption issues you've had earlier and be done with it. Maybe add a note that some architectures might need a special alignement and fail if it is not the case but nothing really specific. -- Michal Hocko SUSE Labs -- To unsubscribe from this list: send the line "unsubscribe linux-api" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
