linux-ext4 - Re: [PATCH 08/11] vfs: Unmap underlying metadata of new data buffers only when buffer is mapped

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Message-ID: <20090528101554.GB27978@skywalker>
Date:	Thu, 28 May 2009 15:45:54 +0530
From:	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>
To:	Jan Kara <jack@...e.cz>
Cc:	LKML <linux-kernel@...r.kernel.org>, npiggin@...e.de,
	linux-ext4@...r.kernel.org
Subject: Re: [PATCH 08/11] vfs: Unmap underlying metadata of new data
	buffers only when buffer is mapped

On Thu, May 28, 2009 at 11:44:34AM +0200, Jan Kara wrote:
> On Wed 27-05-09 21:05:59, Aneesh Kumar K.V wrote:
> > On Wed, May 27, 2009 at 03:01:05PM +0200, Jan Kara wrote:
> > > When we do delayed allocation of some buffer, we want to signal to VFS that
> > > the buffer is new (set buffer_new) so that it properly zeros out everything.
> > > But we don't have the buffer mapped yet so we cannot really unmap underlying
> > > metadata in this state. Make VFS avoid doing unmapping of metadata when the
> > > buffer is not yet mapped.
> > > 
> ...
> > > @@ -2683,7 +2685,7 @@ int nobh_write_begin(struct file *file, struct address_space *mapping,
> > >  			goto failed;
> > >  		if (!buffer_mapped(bh))
> > >  			is_mapped_to_disk = 0;
> > > -		if (buffer_new(bh))
> > > +		if (buffer_new(bh) && buffer_mapped(bh))
> > >  			unmap_underlying_metadata(bh->b_bdev, bh->b_blocknr);
> > >  		if (PageUptodate(page)) {
> > >  			set_buffer_uptodate(bh);
> > 
> > Both xfs and ext4 return mapped delay buffer_head when we do a get_block
> > with delayed allocation in write_begin phase.
>   Yeah, I knew about ext4 doing this. Thanks for pointing this out. I
> wanted to trigger a separate discussion about this and similar problems -
> the current state of buffer bits is quite messy (I think Ted complained
> about this as well recently) and we should somehow clean it up.
>   In this particular case: What's the point in returning the buffer mapped?
> It does not make any sence logically (block *does not* have any physical
> location assigned) and technically you have to map it to some fake block
> and later remap it correctly when you do block allocation. So maybe I'm
> missing some good reason but from what I can see, it just does not make
> sence...

Marking it mapped make sure we don't do multiple get_block calls for
every write. For each write in write_begin path we do a get_block
call if the buffer is not mapped. (__block_prepare_write have more
details.)

-aneesh
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