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SubjectRe: OOPS in perf_mmap_close()
On Thu, 23 May 2013, Peter Zijlstra wrote:

> On Thu, May 23, 2013 at 10:10:36AM -0400, Vince Weaver wrote:
> >
> > I can confirm your patch avoids the oops on my machine.
> >
> > It does lead to interesting behavior if I run the sample program
> > multiple times (with added printfs):
> >
> > vince@core2:~$ ./perf_mmap_close_bug
> > mmap1=0x7f06a6e90000
> > mmap2=0x7f06a6e7f000
> > vince@core2:~$ ./perf_mmap_close_bug
> > mmap1=0x7f878a138000
> > mmap2=0x7f878a127000
> > vince@core2:~$ ./perf_mmap_close_bug
> > mmap1=0xffffffffffffffff
> > Error opening fd2 Invalid argument
> >
> > and then it never successfully completes again. Is this unexpected
> > behavior?
>
> Sounds weird to me, I'll see if I can reproduce/understand.

I tracked this down in case you haven't already.

The problem is that in the kernel patched
with your patch locked_vm is getting decremented twice in the sample code
and going negative. I'm not sure why this isn't a problem until the
third time through. Here are my crude debug printk
results from kernel/events/core.c

[ 28.684862] user_extra: 17 user_lock_limit: 129
[ 28.698458] user_locked: 17 locked_vm: 0 user_extra 17
[ 28.713853] locked: 0 locked_vm: 0 pinned_vm: 0 extra: 0 lock_limit: 16
[ 28.733728] perf_mmap: locked_vm: 17
[ 28.744509] mmap_close: locked_vm=0
[ 28.754939] mmap_close: locked_vm=-17
[ 29.472741] user_extra: 17 user_lock_limit: 129
[ 29.486332] user_locked: 0 locked_vm: -17 user_extra 17
[ 29.501996] locked: 0 locked_vm: 0 pinned_vm: 0 extra: 0 lock_limit: 16
[ 29.521874] perf_mmap: locked_vm: 0
[ 29.532400] mmap_close: locked_vm=-17
[ 29.543352] mmap_close: locked_vm=-34
[ 30.028236] user_extra: 17 user_lock_limit: 129
[ 30.041835] user_locked: -17 locked_vm: -34 user_extra 17
[ 30.058018] extra: -275 user_locked: -17 user_lock_limit: 258
[ 30.075232] locked: -275 locked_vm: 0 pinned_vm: 0 extra: -275 lock_limit: 16

Vince


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