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 12:01:18 -0500
From:	Eric Sandeen <sandeen@...hat.com>
To:	Theodore Tso <tytso@....edu>
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

Theodore Tso wrote:
> 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.

I'm sorry.  Poor choice of fonts, or something, I read "-0" not "~0" and
wondered what on earth you were thinking.  ;)

-Eric
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