On 12/04/2017 11:08 PM, Michal Hocko wrote: > 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. > Sure, I can drop the "how to use this safely" section. It seemed like a good idea at the time... :) thanks, John Hubbard NVIDIA -- 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
