linux-ext4 - Re: ext4: Can we talk about bforget() and metadata blocks

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Message-ID: <20090911172125.GA10155@skywalker.linux.vnet.ibm.com>
Date:	Fri, 11 Sep 2009 22:51:25 +0530
From:	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>
To:	Theodore Tso <tytso@....edu>
Cc:	Curt Wohlgemuth <curtw@...gle.com>, linux-ext4@...r.kernel.org
Subject: Re: ext4: Can we talk about bforget() and metadata blocks

On Thu, Sep 10, 2009 at 02:58:26PM -0400, Theodore Tso wrote:
> On Thu, Sep 10, 2009 at 09:54:35PM +0530, Aneesh Kumar K.V wrote:
> > 
> > But how would it work for fsync ? I mean 
> > 
> > I would expect for no journal mode ext4_sync_file  should be doing
> > simple_fsync(). That should be forcing the metadata buffer_heads
> > via sync_mapping_buffers. And if we reuse these meta buffers we
> > drop them the inode->mapping->private_list using bforget.
> > 
> > But I don't see any of the above in code
> 
> Aneesh, you're addressing a different problem than the one that Curt
> were trying to deal with this patch.  The problem we are worry about
> is one where an inode's extent tree or indirect blocks are modified
> right before the inode is deleted, and then one or more of those
> metadata blocks get reallocated and written right away (most likely
> this will happen via an O_DIRECT write), and then, because we didn't
> use bforget(), the dirty metadata block in the buffer cache would get
> written out, overwriting the O_DIRECT block.
> 
> What you're worrying about, is a different issue.  You're concerned
> about the fact that since we are not associating an inode's extent
> tree or indirect blocks with the inode, those blocks won't get forced
> out to disk on an fsync() in ext4 no-journal mode.  This may not be a
> big deal for applications which expect to recover from an unclean
> using mke2fs (and thus probably don't use fsync in any case), but
> here's a patch to deal with the problem you've raised.
> 
>        	       	       	    		       - Ted

But the patch you posted is using bforget which is removing the
buffer_head from the inode->mapping->private_list. What i am
trying to figure out is where does the buffer_head getting added
to the private_list. ?

-aneesh
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