linux-ext4 - Re: [RFC PATCH] mark buffer_head mapping preallocate area as new during write

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Message-ID: <20090428124821.GJ22104@mit.edu>
Date:	Tue, 28 Apr 2009 08:48:21 -0400
From:	Theodore Tso <tytso@....edu>
To:	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>
Cc:	Mingming Cao <cmm@...ibm.com>, sandeen@...hat.com,
	linux-ext4@...r.kernel.org
Subject: Re: [RFC PATCH] mark buffer_head mapping preallocate area as new
	during write_begin with delayed allocation

On Tue, Apr 28, 2009 at 03:01:45PM +0530, Aneesh Kumar K.V wrote:
> 
> Looking at the source again i guess setting just b_dev is not enough.
> unmap_underlying_metadata looks at the mapping block number, which we
> don't have in case on unwritten buffer_head. How about the below patch ?
> It involve vfs changes. But i guess it is correct with respect to the
> meaning of BH_New (Disk mapping was newly created by get_block). I guess
> BH_New implies BH_Mapped.

Argh.  So we have multiple problems going on here.  One is the
original problem, namely that of a partial write into an preallocated
block can leave garbage behind in that unitialized block.

The other problem seems to be in the case of a delayed allocation
write, where we return a buffer_head which is marked new, and this
causes block_prepare_write() to call unmap_underlying_metadata(dev, 0).

In theory this could cause problems if we try installing a new
bootloader in the filesystem's boot block while there's a delayed
writes happening in the background, since we could end up discarding
the write to the boot sector.  We've lived with this for quite a wihle
though.

My concern with making the fs/buffer.c changes is that we need to make
sure it doesn't break any of the other filesystems, so that's going to
make it hard to try to slip this with 2.6.30-rc4 nearly upon us.
(Silly question; why doesn't XFS get caught by this?) 

So the question is do we try to fix both bugs with one patch, and very
likely have to wait until 2.6.31 before the patch is incorporated?  Or
do we fix the second bug using an ext4-only fix, with the knowledge
that post 2.6.30, we'll need undo most of it and fix it properly with
a change that involves fs/buffer.c?

My preference is for the former, unless we belive the 2nd bug is
serious enough that we really need to address it ASAP (in which case
we have a lot of work ahead of us in terms of coordinating with the
other filesystem developers).   What do other folks think?

      		 		     	      	    - Ted
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