In message <Pine.LNX.4.64.0806291244440.32708@xxxxxxxxxxx>, Hugh Dickins writes: > 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 Hugh, yes, the only place in fsstack where i_size is used is to copy the lower i_size to the upper one verbatim. If this assumption is incorrect for some lower file systems, then stackable file systems in general may have a problem with this assumption; in that case, we'll need an alternative way to "interpret" the lower i_size, and report the i_size in the upper inode (and hence to the user). Is there such an alternative? BTW, ecryptfs may have a problem with this, b/c it uses i_size_read/write b/t the lower and upper inodes. If some filesystems have a different interpretation for i_size, then stacking ecryptfs on top of them could be an issue. Mike? Erez. -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
