linux-ext4 - Re: [PATCH -V4 2/2] ext4: Use -1 as the fake block number for delayed new buffer

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Date:	Wed, 29 Apr 2009 12:52:10 -0400
From:	Theodore Tso <tytso@....edu>
To:	Eric Sandeen <sandeen@...hat.com>
Cc:	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>,
	cmm@...ibm.com, linux-ext4@...r.kernel.org
Subject: Re: [PATCH -V4 2/2] ext4: Use -1 as the fake block number for
	delayed new buffer_head

On Wed, Apr 29, 2009 at 10:37:34AM -0500, Eric Sandeen wrote:
> Theodore Tso wrote:
> > On Wed, Apr 29, 2009 at 10:17:21AM +0530, Aneesh Kumar K.V wrote:
> >> Block number '0' should not be used as the fake block number for
> >> the delayed new buffer. This will result in vfs calling umap_underlying_metadata for
> >> block number '0'. So  use -1 instead.
> > 
> > sector_t is an unsigned type, so we probably want to use ~0 instead of
> > -1.  I can fix this up before we apply into the patch queue.
> 
> I don't think that helps.  The point is to have a block number which is
> invalid, therefore won't get unmapped or accidentally written to ...

This is more of a type-safety thing to eliminate compiler warnings.
We could use something like s_blocks_count instead, which has less
chance of wrapping, but by the time we get to the bh level, the risk
of wrapping should be minimal, and ~0 (or -1) is more distinctive when
debugging/tracing.

					- Ted
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