On Fri, Feb 17, 2017 at 12:02 PM, Linus Torvalds
[off-list ref] wrote:

On Fri, Feb 17, 2017 at 6:13 AM, Kirill A. Shutemov
[off-list ref] wrote:

quoted

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

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