Re: msync(2) bug(?), returns AOP_WRITEPAGE_ACTIVATE to userland

[Hugh Dickins <hugh@veritas.com>]

On Mon, 5 Nov 2007, Dave Hansen wrote:
> 
> Actually, I think your s/while/if/ change is probably a decent fix.

Any resemblance to a decent fix is purely coincidental.

> Barring any other races, that loop should always have made progress on
> mnt->__mnt_writers the way it is written.  If we get to:
> 
> >                 lock_and_coalesce_cpu_mnt_writer_counts();
> ----------------->HERE
> >                 mnt_unlock_cpus();
> 
> and don't have a positive mnt->__mnt_writers, we know something is going
> badly.  We WARN_ON() there, which should at least give an earlier
> warning that the system is not doing well.  But it doesn't fix the
> inevitable.  Could you try the attached patch and see if it at least
> warns you earlier?

Thanks, Dave, yes, that gives me a nice warning:

leak detected on mount(c25ebd80) writers count: -65537
WARNING: at fs/namespace.c:249 handle_write_count_underflow()
 [<c0103486>] show_trace_log_lvl+0x1b/0x2e
 [<c01034b6>] show_trace+0x16/0x1b
 [<c0103589>] dump_stack+0x19/0x1e
 [<c0171906>] handle_write_count_underflow+0x4c/0x60
 [<c0171983>] mnt_drop_write+0x69/0x8e
 [<c0160211>] __fput+0xff/0x162
 [<c016010d>] fput+0x2e/0x33
 [<c01b8f63>] unionfs_file_release+0xc2/0x1c5
 [<c01601a1>] __fput+0x8f/0x162
 [<c016010d>] fput+0x2e/0x33
 [<c015ec9d>] filp_close+0x50/0x5d
 [<c015ed1e>] sys_close+0x74/0xb4
 [<c01026ce>] sysenter_past_esp+0x5f/0x85

and the test then goes quietly on its way instead of hanging.  Though
I imagine, with your patch or mine, that it's then making an unfortunate
frequency of calls to lock_and_coalesce_longer_name_than_I_care_to_type
thereafter.  But it's hardly your responsibility to optimize for bugs
elsewhere.

The 2.6.23-mm1 tree has MNT_USER at 0x200, so I adjusted your flag to
#define MNT_IMBALANCED_WRITE_COUNT	0x400 /* just for debugging */

> 
> I have a decent guess what the bug is, too.  In the unionfs code:

I'll let Erez take it from there...

Hugh
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