Re: [LTP] mmapstress03 weirdness? (fwd)

On Tue, 22 Sep 2009, Benjamin Herrenschmidt wrote:

On Mon, 2009-09-21 at 15:40 +0200, Geert Uytterhoeven wrote:

quoted

With 32-bit userland, this boils down to:
| mmap addr 0x7fff0000 size 0x7fff0000
| mmap returned 0x7fff0000

i.e. mmap() succeeds, but (1) the test expects it to fail, so the test returns
TFAIL, but (2) ltp-pan still reports that the tests passed?

What is the output of /proc/<pid>/maps after that mmap ?

| 00100000-00120000 r-xp 00100000 00:00 0                                  [vdso]
| 0f470000-0f5d0000 r-xp 00000000 03:03 56852565                           /lib/libc-2.5.so
| 0f5d0000-0f5e0000 r--p 00160000 03:03 56852565                           /lib/libc-2.5.so
| 0f5e0000-0f5f0000 rw-p 00170000 03:03 56852565                           /lib/libc-2.5.so
| 0ffc0000-0ffe0000 r-xp 00000000 03:03 56852482                           /lib/ld-2.5.so
| 0ffe0000-0fff0000 r--p 00010000 03:03 56852482                           /lib/ld-2.5.so
| 0fff0000-10000000 rw-p 00020000 03:03 56852482                           /lib/ld-2.5.so
| 10000000-10010000 r-xp 00000000 03:03 65571126                           /tmp/a.out
| 10010000-10020000 rw-p 00000000 03:03 65571126                           /tmp/a.out
| 7fff0000-fffe0000 rw-s 00000000 00:09 5580806                            /dev/zero (deleted)

I.e. the big mmap() took out the stack mapping, which was previously at:

| ffa00000-ffb50000 rw-p ffa00000 00:00 0                                  [stack]

With a 64-bit kernel, 32-bit userspace has access to the entire 4G
address space, so mapping 2G-64k at the 2G-64k point can work, provided
you aren't overlapping an existing mapping such as the stack.

quoted

In addition, sometimes mmapstress03 fails due to SEGV. I created a small test
program that just does the above mmap(), and depending on the distro and what
else I print later it crashes with a SEGV, too. Probably this happens because
the mmap() did succeed, and corrupted some existing mappings, cfr. the notes
for MAP_FIXED:

That's possible.

quoted

       MAP_FIXED
              Don’t  interpret  addr  as  a hint: place the mapping at exactly
              that address.  addr must be a multiple of the page size.  If the
              memory  region  specified  by addr and len overlaps pages of any
              existing mapping(s), then the overlapped part  of  the  existing
              mapping(s)  will  be discarded.  If the specified address cannot
              be used, mmap() will fail.  Because requiring  a  fixed  address
              for  a  mapping is less portable, the use of this option is dis‐
              couraged.

Yeah, I suppose the test might be wiping out its own stack for example

Indeed.

IE. I think that test is just bogus :-)

quoted

JFYI, with 64-bit userland, this boils down to:

| mmap addr 0x7fffffffffff0000 size 0x7fffffffffff0000
| mmap returned 0xffffffffffffffff

i.e. mmap() fails as expected, and the test succeeds.

Right because on 64-bit userspace, you only are allowed something like
16T of address space.

quoted

Does all of this sound OK?
Thanks for your comments!

Yes, I think so far, it's just bogus tests :-)

Thanks for the confirmation, Segher and Ben!

With kind regards,

Geert Uytterhoeven
Software Architect
Techsoft Centre

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