On Tue, Dec 06, 2016 at 10:20:20PM +0100, Arnd Bergmann wrote:

On Wednesday, December 7, 2016 12:24:40 AM CET Yury Norov wrote:

quoted

3. Introduce new mmap64() syscall like this:
sys_mmap64(void *addr, size_t len, int prot, int flags, int fd, struct off_pair *off);
(The pointer here because otherwise we have 7 args, if simply pass off_hi and
off_lo in registers.)

This wouldn't have to be a pair, just a pointer to a 64-bit number.

quoted

With new 64-bit interface we can deprecate mmap2(), and generalize all
implementations in kernel.

I think we can discuss it because 64-bit is the default size for off_t 
in all new 32-bit architectures. So generic solution may take place.

The last question here is how important to support offsets bigger than
2^44 on 32-bit machines in practice? It may be a case for ARM64 servers,
which are looking like main aarch64/ilp32 users. If no, we can leave
things as is, and just do nothing.

If there is a use case for larger than 16TB offsets, we should add
the call on all architectures, probably using your approach 3. I don't
think that we should treat it as anything special for arm64 though.

From this point of view, 16+TB offset is a matter of 16+TB storage,

and it's more than real. The other consideration to add it is that
we have 64-bit support for offsets in syscalls like sys_llseek().
So mmap64() will simply extend this support.

I can prepare this patch. Some implementation details I'd like to
clarify:
Syscall declaration:
SYSCALL_DEFINE6(mmap64, unsigned long, addr, unsigned long, len,
                unsigned long, prot, unsigned long, flags,
                unsigned long, fd, unsigned long long *, offset);

sys_mmap64() deprecates sys_mmap2(), and __ARCH_WANT_MMAP2 is
introduced to keep it enabled for all existing architectures.
All modern arches (aarch64/ilp32 is the first candidate) will have
mmap64() only. The example is set/getrlimit() or renameat() drop
patches (b0da6d44).
                                
On GLIBC side, __OFF_T_MATCHES_OFF64_t will wire mmap() from
linux/generic/wordsize32/mmap.c to mmap64() from linux/mmap64.c. 

mmap64() will first try __NR_mmap64, and if not defined, or ENOSYS
is returned, __NR_mmap2 will be called. This is to let userspace that
supports both mmap2() and mmap64() have full 64-bit offset support, not
44-bit one.

For __NR_mmap2 case, I'd also add the check against offsets more than
2^44, and set errno to EOVERFLOW in that case.

Any thoughts?

Yury.