Re: [PATCH] fsstack: fsstack_copy_inode_size locking

[Hugh Dickins <hugh@veritas.com>]

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