linux-kernel - Re: [RFC PATCH 0/6] Union mount core rewrite v1

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Date:	Wed, 03 Mar 2010 18:28:49 +0100
From:	Miklos Szeredi <miklos@...redi.hu>
To:	Valerie Aurora <vaurora@...hat.com>
CC:	viro@...iv.linux.org.uk, hch@...radead.org,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	vaurora@...hat.com
Subject: Re: [RFC PATCH 0/6] Union mount core rewrite v1

On Tue,  2 Mar 2010, Valerie Aurora wrote:
> This is a major rewrite of parts of union mounts, in particular the
> pathname lookup code.  For more info about union mounts, see:
> 
> http://valerieaurora.org/union/
> 
> The previous code had two important problems fixed in this series:
> 
> - On file open, is_unionized() grabs vfsmount lock and walks up the
>   mount tree even for non-union mounts.
> 
> - Pathname lookup required three cut-n-pasted versions of two complex
>   functions, one for each of cached/real/"hashed" lookups.

Looks much better :)

> This rewrite reduces the additional cost of a non-union lookup in a
> CONFIG_UNION_MOUNT kernel to either 1 or 2 mount flag tests (but adds
> the requirement that file systems be unioned only at their root
> directories).  This rewrite implements lookup with one lookup_union()
> function for all types of lookups.
> 
> This posted patch series includes only the union lookup, mount, and
> readdir patches and not the relatively uncontroversial whiteout and
> fallthru code.

Special inode/dentry flags (whiteout, fallthrough, opaque) are not
trivially the right solution:

 - they are invisible from userspace, new APIs are necessary to manipulate them
 - they are difficult to support on network filesystems
 - they are not useful for anything other than union mounts/filesystems

Extended attributes are a more standard way to add such info to files.
Hard links would allow sharing a single inode for all whiteout entries
and one for all fallthrough entries.

Have these options been considered as an alternative?

Thanks,
Miklos
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