Re: OOPS in perf_mmap_close()

[Vince Weaver]

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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