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Date: Fri, 7 Jan 2011 02:29:34 -0500
From: Christoph Hellwig <hch@infradead.org>
To: Nick Piggin <npiggin@kernel.dk>
Cc: Christoph Hellwig <hch@infradead.org>, linux-fsdevel@vger.kernel.org,
        linux-kernel@vger.kernel.org,
        Andrew Morton <akpm@linux-foundation.org>
Subject: Re: [patch 8/8] fs: add i_op->sync_inode
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On Fri, Jan 07, 2011 at 02:24:30AM -0500, Christoph Hellwig wrote:
> On Fri, Jan 07, 2011 at 03:47:34PM +1100, Nick Piggin wrote:
> > No, you misunderstand 1. I am saying they should be treated as
> > WB_SYNC_NONE.
> > 
> > In fact 2 would cause much more IO, because dirty writeout would
> > never clean them so it will just keep writing them out. I don't
> > know how 2 could be feasible.
> 
> WB_SYNC_NONE means ->write_inode behaves non-blocking.  That is
> we do not block on memory allocations, and we do not take locks
> blocking.  Most journaling filesystems currently take the easy
> way out an make it a no-op due to that, but take a look at XFS
> how complicated it is to avoid the blocking if you want a non-noop
> implementation.

Btw, there's an easy way how we could get this right, in fact
the write_inode in XFS is already trying to do it, it's just the
caller not copying with it:

 - if we can't get locks for a non-blocking ->write_inode we return
   EAGAIN, and the callers sets the dirty bits again.