[patch 0/4] MAP_NOZERO v2 - VM_NOZERO/MAP_NOZERO early summer madness

Davide Libenzi
2007-06-28T18:49:35Z

I was using oprofile to sample some userspace code I am working on,
and I was continuosly noticing clear_page in the top three entries
of the oprofile logs.
Also, a simple kernel build, in my Dual Opteron with 8GB of RAM,
shows clear_page as the first kernel entry, second only to the
userspace the cc1 and as.
Most of the userspace code uses malloc() (and anonymous mappings) in
such a way that the memory returned via kernel->glibc is immediately
written soon after. The POSIX malloc() definition itself also, does
not require the returned memory to be zeroed (as calloc() does).
So I implemented a rather quick hack that introduces a new mmap() flag
MAP_NOZERO (only valid for anonymous mappings) and the vma counter-part
VM_NOZERO. Also, a new sys_brk2() has been introduced to accept a new
flags parameter. A brief description of the patches follows in the next
emails.
I first hacked Val's ebizzy to accept a new '-N' flag to make use of
MAP_NOZERO:

http://infohost.nmt.edu/~val/patches/ebizzy.tar.gz
http://www.xmailserver.org/ebizzy-nzmmap-0.3.diff

On my box, ebizzy performance jumped up from 10% to 15%.
The userspace code I am working on (uses malloc() quite heavily), saw
a performance jump of around 14%.
In both cases, clear_page dropped way down in the oprofile logs.
I then coded quick (and rather ugly) hacks for glibc and gcc to
make them use the new features (MAP_NOZERO and sys_brk2()):

http://www.xmailserver.org/glibc-nzmalloc-tweaks
http://www.xmailserver.org/gcc-nozero-hack

I then tried a 2.6.22-rc5 kernel build using the newly built glibc
and gcc (with and w/out no-zero enabling options/env-vars), and
when using the no-zero mode, clear_page went way down in the oprofile
logs and build time dropped of about 2.5% to 3%.
I did not have time (and will) to tweak as and ld also.
These are some test utilities to verify the no-zero behaviour of MAP_NOZERO
(and sys_brk2()):

http://www.xmailserver.org/nzmmap-test.c
http://www.xmailserver.org/nzmalloc-test.c
http://www.xmailserver.org/smiffy.c

To run nzmalloc-test you need a patched glibc (using glibc-nzmalloc-tweaks).
The smiffy one, should be run under a user that has no other processes
running and that owns no files on the system, and it verifies that all the
pages it gets from the kernel are zeroed (otherwise "Houston, we have a problem ...").
It is running on my system w/out barfing by more than two days.
How crazy is that?

- Davide

ChangeLog:

* Version 2

o Reusing _mapcount instead of adding a new field in the page struct
o Added a fix for a setuid+exec/ptrace race (Andy spotted)