[Linux-cluster] Re: GFS

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

Hi, 

On Wed, Aug 10, 2005 at 11:18:48PM +0300, Pekka J Enberg wrote:
> > You, however, don't maintain the same level of data consistency when reads
> > and writes are from other filesystems as they use ->nopage.

Mark Fasheh writes:
> I'm not sure what you mean here...

Reading and writing from other filesystems to a GFS2 mmap'd file
does not walk the vmas. Therefore, data consistency guarantees
are different: 

- A GFS2 filesystem does a read that writes to a GFS2 mmap'd
 file -> we take all locks for the mmap'd buffer in order and
 release them after read() is done. 

- A ext3 filesystem, for example, does a read that writes to
 a GFS2 mmap'd file -> we now take locks one page at the
 time releasing them before we exit ->nopage(). Other nodes
 are now free to write to the same GFS2 mmap'd file. 

Or am I missing something here? 

On Wed, Aug 10, 2005 at 11:18:48PM +0300, Pekka J Enberg wrote:
> > Fixing this requires a generic vma walk in every write() and read(), no?
> > That doesn't seem such an hot idea which brings us back to using ->nopage
> > for taking the locks (but now the deadlocks are back).

Mark Fasheh writes:
> Yeah if you look through mmap.c in ocfs2_fill_ctxt_from_buf() we do this...
> Or am I misunderstanding what you mean?

If are doing write() or read() from some other filesystem, we don't walk the 
vmas but instead rely on ->nopage for locking, right? 

                  Pekka