From: Alex Tomas <bzzz@sun.com>
Subject: Re: Potential bug in mballoc --- reusing data blocks before txn	commit
Date: Tue, 30 Sep 2008 17:12:12 +0400
Message-ID: <48E225AC.9090208@sun.com>
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Cc: Andreas Dilger <adilger@sun.com>, linux-ext4@vger.kernel.org
To: Theodore Tso <tytso@mit.edu>
In-reply-to: <20080930130247.GM10831@mit.edu>
Sender: linux-ext4-owner@vger.kernel.org

Theodore Tso wrote:
> On Tue, Sep 30, 2008 at 08:35:21AM +0400, Alex Tomas wrote:
>> why we need a tree? at least for the purpose of keeping blocks unavailable
>> we'd need just a list as at commit we free them all.
> 
> For ext4, the only reason to use a tree would be to allow us to merge
> deleted extents.  This might not be worth the complexity, though, I
> admit it.

strictly speaking, extents code should have merged them at allocation time.

>>> The other thing which I should check is that if we are using this
>>> scheme, I think we shouldn't need to keep the shadow copy of the block
>>> bitmap buffers any more.  I would imagine we still need them for the
>>> inode bitmaps, for the same reason, though.
>> shadow copy holds preallocated blocks
> 
> Are we talking about the same thing?  I was referring to the
> jh->b_committed_data, which isn't used by mballoc at all.

oops. I meant in-core bitmap mballoc generates. if there is intention
to get rid of old allocator (balloc.c), then we don't need b_committed_data.

btw, I've just remembered why I decided don't protect data from reallocation:
in data=writeback one can get block with stale data easily. and many people
(to my knowledge) were using data=writeback as performing better.

thanks, Alex