Sigh... apologies for the HTML. Trying again...

On Mon, Feb 20, 2017 at 9:21 PM, Michael Pratt [off-list ref] wrote:

On Fri, Feb 17, 2017 at 3:02 PM, Andy Lutomirski [off-list ref] wrote:

quoted

On Fri, Feb 17, 2017 at 1:01 PM, Linus Torvalds
[off-list ref] wrote:

quoted

On Fri, Feb 17, 2017 at 12:12 PM, Andy Lutomirski [off-list ref] wrote:

quoted

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.

quoted

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.

--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@kvack.org.  For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>