linux-kernel - Re: [PATCH] fsstack: fsstack_copy_inode

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Date:	Sun, 29 Jun 2008 12:56:24 +0100 (BST)
From:	Hugh Dickins <hugh@...itas.com>
To:	Christoph Hellwig <hch@...radead.org>
cc:	Erez Zadok <ezk@...sunysb.edu>,
	Andrew Morton <akpm@...ux-foundation.org>, mhalcrow@...ibm.com,
	hooanon05@...oo.co.jp, viro@...iv.linux.org.uk,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] fsstack: fsstack_copy_inode_size locking

On Sun, 29 Jun 2008, Christoph Hellwig wrote:
> 
> Btw, I hope fsstack doesn't rely on i_size having any particular
> meaning.  As far as the VFS is concerned i_size is field only used by
> the filesystem (or library routines like generic_file_*).

Interesting point.  I can't speak for fsstack itself (I'm not even
sure if it's anything beyond fs/stack.c and the tag I used to identify
where this patch lies); but certainly fs/stack.c doesn't use i_size
for anything, just duplicates it from the lower filesystem.

unionfs (which I think you don't care for at all in general) does
look as if it assumes it's the lower file size in a few places,
when copying up or truncating.  Isn't that reasonable?  Wouldn't
users make the same assumption?

Or are you saying that filesystems which don't support the usual
meaning of inode->i_size (leave it 0?) would supply their own
equivalent to vmtruncate() if they support truncation, and their
own getattr which fills in stat->size from somewhere else.

Yes, I think you are saying that: unionfs may not play well with them.

Hugh
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