# Re: \[PATCHv3 33/33\] mm, x86: introduce PR\_SET\_MAX\_VADDR and PR\_GET\_MAX\_VADDR

- List: linux-api
- 1 message, 1 author
- Page: https://ratatoskr.run/linux-api/2017/02/6483403/t

## Re: \[PATCHv3 33/33\] mm, x86: introduce PR\_SET\_MAX\_VADDR and PR\_GET\_MAX\_VADDR

- From: Michael Pratt \<hidden\>
- Date: 2017-02-21 06:01:09 
- Message: https://ratatoskr.run/linux-api/2017/02/6483403.md

```text
On Mon, Feb 20, 2017 at 9:21 PM, Michael Pratt [ref] wrote:

> On Fri, Feb 17, 2017 at 3:02 PM, Andy Lutomirski [ref]
> wrote:
> > On Fri, Feb 17, 2017 at 1:01 PM, Linus Torvalds
> > [ref] wrote:
> >> On Fri, Feb 17, 2017 at 12:12 PM, Andy Lutomirski [ref]
> wrote:
> >>>
> >>> At the very least, I'd want to see
> >>> MAP_FIXED_BUT_DONT_BLOODY_UNMAP_ANYTHING.  I *hate* the current
> >>> interface.
> >>
> >> That's unrelated, but I guess w could add a MAP_NOUNMAP flag, and then
> >> you can use MAP_FIXED | MAP_NOUNMAP or something.
> >>
> >> But that has nothing to do with the 47-vs-56 bit issue.
> >>
> >>> How about MAP_LIMIT where the address passed in is interpreted as an
> >>> upper bound instead of a fixed address?
> >>
> >> Again, that's a unrelated semantic issue. Right now - if you don't
> >> pass in MAP_FIXED at all, the "addr" argument is used as a starting
> >> value for deciding where to find an unmapped area. But there is no way
> >> to specify the end. That would basically be what the process control
> >> thing would be (not per-system-call, but per-thread ).
> >>
> >
> > What I'm trying to say is: if we're going to do the route of 48-bit
> > limit unless a specific mmap call requests otherwise, can we at least
> > have an interface that doesn't suck?
>

I've got a set of patches that I've meant to send out as an RFC for a while
that tries to address userspace control of address space layout and covers
many of these ideas.

There is a new syscall and set of prctls for controlling the "mmap layout"
(i.e., get_unmapped_area search range) that look something like this:

struct mmap_layout {
unsigned long start;
unsigned long end;
/*
* These are equivalent to mmap_legacy_base and mmap_base,
* but are not really needed in this proposal.
*/
unsigned long low_base;
unsigned long high_base;
unsigned long flags;
};

/* For flags */
#define MMAP_TOPDOWN 1

struct layout_mmap_args {
unsigned long addr;
unsigned long len;
unsigned long prot;
unsigned long flags;
unsigned long fd;
unsigned long off;
struct mmap_layout layout;
};

void *layout_mmap(struct layout_mmap_args *args);

int prctl(PR_GET_MMAP_LAYOUT, struct mmap_layout *layout);
int prctl(PR_SET_MMAP_LAYOUT, struct mmap_layout *layout);

The prctls control the default range that mmap and friends will allocate.
For 56-bit user address space, it could default to [mmap_min_addr, 1<<47),
as Linus suggests. Applications that want the full address space can
increase it to cover the entire range.

The layout_mmap syscall allows one-off mappings that fall outside the
default layout, and nicely solves the "MAP_FIXED but don't unmap anything
problem" by passing an explicit range to check without actually setting
MAP_FIXED.

This idea is quite similar to the MAX_VADDR + default get_unmapped_area
behavior ides, just more generalized to give userspace more control over
the ultimate behavior of get_unmapped_area.


PS. Apologies if my email client screwed up this message. I didn't have
this thread in my client and have tried to import it from another account.
```