linux-ext4 - Re: [PATCH] Rearrange i_flags to be consistent with FS_IOC

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Message-ID: <20100706001032.GA25018@dastard>
Date:	Tue, 6 Jul 2010 10:10:32 +1000
From:	Dave Chinner <david@...morbit.com>
To:	David Howells <dhowells@...hat.com>
Cc:	viro@...IV.linux.org.uk, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-ext4@...r.kernel.org
Subject: Re: [PATCH] Rearrange i_flags to be consistent with FS_IOC_GETFLAGS

On Mon, Jul 05, 2010 at 04:43:19PM +0100, David Howells wrote:
> Rearrange the constituent bits of i_flags to be consistent with the flag values
> returned by the FS_IOC_GETFLAGS ioctl where flags with common meanings occur.
> Otherwise pack the bits of i_flags as low as possible so that RISC CPUs can
> use small instructions.
> 
> This allows those filesystems that use i_flags (Ext2/3/4) to simplify their
> get/set flags routines.

This seems like a nasty landmine just waiting for someone to step
on.  The S_* flags are VFS level flags used by all filesytsems, not
just extN, and have a history of changing values and meaning as uses
come and go.  This code forces the VFS to use certain flag values
that *match on-disk values* of ext2/3/4.

Further, it opens up the possibility that further down the track
someone modifies S_* flag values (say when adding some XFS
functionality) and corrupts extN filesystems all over the place.
There isn't even compiler guards to catch someone modifying the S_*
flags in a way that makes it incompatible with the extN on-disk
definitions.

Cheers,

Dave.
-- 
Dave Chinner
david@...morbit.com
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