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

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Message-ID: <CAJfpegtpj7s5gOe3k=47PMzKC71ndzg7N2kYRdtRbGXwkA_2Mg@mail.gmail.com>
Date:	Wed, 29 May 2013 10:12:12 +0200
From:	Miklos Szeredi <miklos@...redi.hu>
To:	Zach Brown <zab@...hat.com>
Cc:	Linux-Fsdevel <linux-fsdevel@...r.kernel.org>,
	Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Al Viro <viro@...iv.linux.org.uk>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Christoph Hellwig <hch@...radead.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	David Howells <dhowells@...hat.com>
Subject: Re: [RFC PATCH] vfs: add permute operation

On Tue, May 28, 2013 at 7:36 PM, Zach Brown <zab@...hat.com> wrote:
> 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()?

It's not just plain muti-rename, but a cyclic one.  Maybe cyclic_rename()?

>> +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?

AFAICS, I cannot make the compare function transitive, e.g.:  A is
descendant of C but B is unrelated.  Then what should cmp(A, B) and
cmp(B, C) return?

>
>> +     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?

I could be more consistent with the BUGs.  Doing it with 3 args is not
necessarily good, since then we can't do loops and the chance of a
copy-paste error is increased.

Thanks,
Miklos
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