Re: [PATCH 02/11] vfs: Add better VFS support for page_mkwrite when blocksize < pagesize

[Christoph Hellwig <hch@infradead.org>]

On Mon, Jul 06, 2009 at 11:08:04AM +0200, Nick Piggin wrote:
> OK, hmm, but I wonder -- most of the time do_truncate will need to
> call notify_change anyway, so I wonder if avoiding the double
> indirection saves us anything? (It requires 2 indirect calls either
> way). And if we call ->setsize from ->setattr, then a filesystem
> which implements its own ->setattr could avoid one of those indirect
> calls. Not so if do_truncate has to call ->setattr then ->setsize.

I don't quite understand what you mean here. In the end there should
be one single indirect call, ->setsize (or whatever it's called by
then).

In the first round we'd split up a helper just for size updates from
notify_change, ala:

int vfs_truncate(struct dentry *dentry, loff_t size, int flags, file)
{
	int error;

	error = security_inode_truncate(dentry, size, flags, file);
	if (error)
		return error;

	if (inode->i_op->setsize) {
		inode->i_op->setsize(dentry, size, flags, file);

	} else {
		<... built up iattr here ...>

		if (inode->i_op->setattr) {	
			down_write(&dentry->d_inode->i_alloc_sem);
			error = inode->i_op->setattr(dentry, attr);
			up_write(&dentry->d_inode->i_alloc_sem);
		} else {
			down_write(&dentry->d_inode->i_alloc_sem);
			error = inode_setattr(inode, attr);
			up_write(&dentry->d_inode->i_alloc_sem);
		}
	}

	if (!error)
		fsnotify_truncate(dentry, size, flags);
	return error;
}

One all filesistem are converted to have a setsize method (either their
own or simple_setsize) the !inode->i_op->setsize case can go away.

Note that the above variant moves taking i_alloc_sem into ->setsize as
it's not required for most filesystems (I think only extN need for
O_DIRECT).

Also the above doesn't deal with killing the SUID/SGID bits yet, we'll
need some good way for that.

Actually it might be better to just pass the iattr to ->setsize to so
we can have the parsing for those arguments once, and that filesystems
can re-use parts of their ->setattr for ->setsize if it's complex enough
(timestamp updates and suid/sgid killing)

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