Re: GFS

["Pekka J Enberg" <penberg@cs.helsinki.fi>]

Hi Zach, 

Zach Brown writes:
> I'll try, briefly.

Thanks for the excellent explanation. 

Zach Brown writes:
> And that's the problem. Because they're acquired in ->nopage they can
> be acquired during a fault that is servicing the 'buf' argument to an
> outer file->{read,write} operation which has grabbed a lock for the
> target file. Acquiring multiple locks introduces the risk of ABBA
> deadlocks. It's trivial to construct examples of mmap(), read(), and
> write() on 2 nodes with 2 files that deadlock.

But couldn't we use make_pages_present() to figure which locks we need, sort 
them, and then grab them? 

Zach Brown writes:
> I brought this up with some people at the kernel summit but no one,
> including myself, considers it a high priority.  It wouldn't be too hard
> to construct a patch if people want to take a look.

I guess it's not a problem as long as the kernel has zero or one cluster 
filesystems that support mmap(). After we have two or more, we have a 
problem. 

The GFS2 vma walk needs fixing anyway, I think, as it can lead to buffer 
overflow (if we have more locks during the second walk). 

              Pekka 

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