Re: [PATCHv3 33/33] mm, x86: introduce PR_SET_MAX_VADDR and PR_GET_MAX_VADDR

From: Andy Lutomirski
Date: Fri Feb 17 2017 - 15:21:30 EST

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

> 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?