On Fri, Feb 17, 2017 at 12:02 PM, Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote: > On Fri, Feb 17, 2017 at 6:13 AM, Kirill A. Shutemov > <kirill.shutemov@xxxxxxxxxxxxxxx> wrote: >> This patch introduces two new prctl(2) handles to manage maximum virtual >> address available to userspace to map. > > So this is my least favorite patch of the whole series, for a couple of reasons: > > (a) adding new code, and mixing it with the mindless TASK_SIZE -> > get_max_addr() conversion. > > (b) what's the point of that whole TASK_SIZE vs get_max_addr() thing? > When use one, when the other? > > so I think this patch needs a lot more thought and/or explanation. > > Honestly, (a) is a no-brainer, and can be fixed by just splitting the > patch up. But I think (b) is more fundamental. > > In particular, I think that get_max_addr() thing is badly defined. > When should you use TASK_SIZE, when should you use TASK_SIZE_MAX, and > when should you use get_max_addr()? I don't find that clear at all, > and I think that needs to be a whole lot more explicit and documented. > > I also get he feeling that the whole thing is unnecessary. I'm > wondering if we should just instead say that the whole 47 vs 56-bit > virtual address is _purely_ about "get_unmapped_area()", and nothing > else. > > IOW, I'm wondering if we can't just say that > > - if the processor and kernel support 56-bit user address space, then > you can *always* use the whole space > > - but by default, get_unmapped_area() will only return mappings that > fit in the 47 bit address space. > > So if you use MAP_FIXED and give an address in the high range, it will > just always work, and the MM will always consider the task size to be > the full address space. At the very least, I'd want to see MAP_FIXED_BUT_DONT_BLOODY_UNMAP_ANYTHING. I *hate* the current interface. > > But for the common case where a process does no use MAP_FIXED, the > kernel will never give a high address by default, and you have to do > the process control thing to say "I want those high addresses". > > Hmm? How about MAP_LIMIT where the address passed in is interpreted as an upper bound instead of a fixed address? --Andy
