linux-kernel - Re: NFS BUG_ON in nfs_do

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Date:	Tue, 28 Apr 2009 06:27:17 +0200
From:	Nick Piggin <npiggin@...e.de>
To:	Trond Myklebust <trond.myklebust@....uio.no>
Cc:	linux-fsdevel@...r.kernel.org, Rince <rincebrain@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, linux-nfs@...r.kernel.org
Subject: Re: NFS BUG_ON in nfs_do_writepage

On Sun, Apr 26, 2009 at 01:55:22PM -0400, Trond Myklebust wrote:
> On Sun, 2009-04-26 at 17:13 +0200, Nick Piggin wrote:
> > This doesn't seem to fix the race, though... on kernels with the
> > race still there, it will just open a window where you can have
> > a dirty pte but the page not written out.
> > 
> > I don't understand.
> 
> I'm just pointing out that the NFS client already calls
> __set_page_dirty_nobuffers() while holding the page lock inside the
> nfs_vm_page_mkwrite() call, so having the VM do it too in the call to
> set_page_dirty_balance() is actually redundant. IOW: as far as the NFS
> code is concerned, we can get rid of the ->set_page_dirty() callback in
> that situation.
> 
> I couldn't find any other places in the VM code where we can have a
> dirty pte without also having called page_mkwrite() (and hence
> __set_page_dirty_nobuffers). As I said, adding a WARN_ON(!PageDirty())
> in ->set_page_dirty() didn't ever trigger any cases where the
> set_page_dirty() was actually setting the dirty bit (except in the case
> where we race with page writeout in do_wp_page() and __do_fault()).
> 
> That's why I believe disabling ->set_page_dirty() is safe here, and will
> in fact suffice to fix the page writeout race.

Ah, no I don't think so because it opens another race where the
pte is dity but the page is marked clean.

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