linux-kernel - Re: [RFC PATCH] vfs: add permute operation

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Date:	Tue, 28 May 2013 10:36:31 -0700
From:	Zach Brown <zab@...hat.com>
To:	Miklos Szeredi <miklos@...redi.hu>
Cc:	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	viro@...iv.linux.org.uk, torvalds@...ux-foundation.org,
	hch@...radead.org, akpm@...ux-foundation.org, dhowells@...hat.com
Subject: Re: [RFC PATCH] vfs: add permute operation

Some quick thoughts:

> Permute the location of files.  E.g.  'permute(A, B, C)' is equivalent to A->B,
> B->C and C->A.  This is essentially a series of renames done as a single atomic
> operation.

Hmm.  Can we choose a more specific name than 'permute'?  To me,
->permute() tells me just as much about the operation as
->do_something().  {multi,bulk,mass}_rename()?  renamev()?

Maybe it's just me.

> to be done as an atomic operation.  We could add whiteout support to filesystem
> ops to perform the creation or removal of whiteouts atomically, but it would
> complicate many filesystem ops needlessly.
> 
> Alternatively we can add a generic permute operation and add whiteout support to
> the VFS which utilizes this to perform the operations atomically.

I certainly like the sound of this.

> +static void sort_parents3(struct dentry **p)
> +void sort_parents(struct dentry **p, unsigned *nump)

Yikes, that's a bunch of fiddly code.  Is it *really* worth all that to
avoid calling the generic sort helpers?

> +	if (WARN_ON(num > PERMUTE_MAX) ||
> +	    WARN_ON(num < 2))
> +		return -EINVAL;

And in other places this is a BUG?  Why not, like the syscall, limit the
arguments to three if we're serious about that limitation?

- z
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