Subject: Re: infinite loop? with mmap, nfs, pwrite, O_DIRECT
From: Trond Myklebust <trond.myklebust@fys.uio.no>
To: Andrew Morton <akpm@osdl.org>
Cc: theonetruekenny@yahoo.com, cel@citi.umich.edu,
       linux-kernel@vger.kernel.org
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	 <1132612974.8011.12.camel@lade.trondhjem.org>
	 <20051121153454.1907d92a.akpm@osdl.org>
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Date: Mon, 21 Nov 2005 18:58:23 -0500
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On Mon, 2005-11-21 at 15:34 -0800, Andrew Morton wrote:
> Trond Myklebust <trond.myklebust@fys.uio.no> wrote:
> >
> >  Anything that calls lock_page() should be avoided in O_DIRECT,
> 
> Why?
> 
> And it's still doing lock_page():
> 
> 	nfs_file_direct_write()
> 	->filemap_fdatawrite()
> 	  ->do_writepages()
> 	    ->nfs_writepages()
> 	      ->generic_writepages()
> 	        ->mpage_writepages()
> 	          ->lock_page()

True.

> > however
> >   we should be able to call invalidate_inode_pages() since that doesn't
> >   wait on the page lock.
> 
> invalidate_inode_pages2() is better.


> And using generic_file_direct_IO() is
> better still, since it handles mmap coherency and only work upon that part
> of the file which is actually undergoing IO.

Unlike local filesystems, we don't want to have to take the i_sem in any
of the direct IO paths. The latter is just a liability as far as
applications are concerned: it doesn't offer any protection for local
data (there _is_ no local data to protect), but gets seriously in the
way of write parallelism.

The only difference I can see between the two paths is the call to
unmap_mapping_range(). What effect would that have?

Cheers,
  Trond

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