linux-ext4 - Re: [PATCH] ext4: add checksum calculation when clearing UNINIT flag

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Message-ID: <20081107135222.GB9543@mit.edu>
Date:	Fri, 7 Nov 2008 08:52:22 -0500
From:	Theodore Tso <tytso@....edu>
To:	Frédéric Bohé <frederic.bohe@...l.net>
Cc:	linux-ext4 <linux-ext4@...r.kernel.org>
Subject: Re: [PATCH] ext4: add checksum calculation when clearing UNINIT
	flag

On Fri, Nov 07, 2008 at 11:22:56AM +0100, Frédéric Bohé wrote:
> From: Frederic Bohe <frederic.bohe@...l.net>
> 
> Block group's checksum need to be re-calculated during the
> initialization of an UNINIT'd group. This fix a race when several
> threads try to allocate a new inode in an UNINIT'd group.

This patch looks sane, and so I'll accept it, but there's a higher
order hiding here ---- why are we initializing the block bitmap in
ext4_new_inode()?  Sure, *most* of the time where we create a new
inode, we'll be needing to allocate a new block, but sometimes we
won't (i.e., when creating a symlink, device file, socket, or a
zero-length regular file).  More seriously, we don't account for the
potential need for an extra journal credit in all of the callers for
ext4_new_inode().  Obviously this doesn't get us in trouble because we
generally massively overestimate the number of journal credits we need
--- but from the point of view of code simplification, maybe code
block to ininitialize the block bitmap in ext4_new_inode() should be
dropped entirely.

We have to do the exact same check in the mballoc.c when we actually
allocate blocks --- and in that case we know we'll be modifying the
block bitmap, so there's no need to first initialize the block bitmap
in ext4_new_inode(), only to need to request to redirty that same
block bitmap in mballoc.c when we are really allocating data for the
inode.

Does that make sense for a future cleanup?

						- Ted
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