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: <20090528135007.GM29199@duck.suse.cz>
Date:	Thu, 28 May 2009 15:50:07 +0200
From:	Jan Kara <jack@...e.cz>
To:	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>
Cc:	Jan Kara <jack@...e.cz>, 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 28-05-09 15:45:54, Aneesh Kumar K.V wrote:
> 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
> > sense...
> 
> 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.)
  Thanks for explanation. But you can always immediately return from
get_block() when you see that buffer has buffer_delay() set if you do not
want to do an allocation from there (e.g. in ext3 I want to do an allocation
in such case for compatibility reasons but that's a separate story).
  Alternatively, if the get_block() call + immediate return bothers you, we
could introduce something like AOP_FLAG_DELALLOC and make
__block_prepare_write call get_block only if buffer is !mapped and !delay
when this flag is set. IMO that would be probably the best. What do you
think?

								Honza
-- 
Jan Kara <jack@...e.cz>
SUSE Labs, CR
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