linux-kernel - Re: [PATCH 00/13] overlay filesystem: request for inclusion (v16)

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Date:	Thu, 14 Mar 2013 11:37:50 +0100
From:	Miklos Szeredi <miklos@...redi.hu>
To:	Al Viro <viro@...iv.linux.org.uk>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Christoph Hellwig <hch@...radead.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Robo Bot <apw@...onical.com>, Felix Fietkau <nbd@...nwrt.org>,
	Neil Brown <neilb@...e.de>,
	Jordi Pujol <jordipujolp@...il.com>, ezk@....cs.sunysb.edu,
	David Howells <dhowells@...hat.com>,
	Sedat Dilek <sedat.dilek@...glemail.com>,
	"J. R. Okajima" <hooanon05@...oo.co.jp>
Subject: Re: [PATCH 00/13] overlay filesystem: request for inclusion (v16)

On Thu, Mar 14, 2013 at 12:19 AM, Al Viro <viro@...iv.linux.org.uk> wrote:
> On Wed, Mar 13, 2013 at 11:09:07PM +0100, Miklos Szeredi wrote:

>> As for same upper on unrelated lower: just don't do it.  As I said, we
>> could enforce this, but I don't think this is top priority.
>
> Tell that to container crowd - they seem to be hell-bent on making everything
> mount-related non-priveleged ;-/

Which is good, but it does need some care.  I'm happy to review those changes.

>> > * ->follow_link():  Why the hell do you bother with struct ovl_link_data???
>> > Just to avoid calling ovl_dentry_real() in ovl_put_link()?
>>
>> Yes, a copy-up between ovl_follow_link and ovl_put_link will break that.
>
> *blink*
>
> Er...  What's wrong with simply unhashing the sucker on copyup if it's
> a symlink?

Nothing, so I'll do that.  Actually we can do that for all except
directory dentries and save some worry.

> BTW, looking at your ovl_copy_up() - you do realize that dget_parent(d)
> does *not* guarantee that returned dentry will remain the parent of d?
> rename() can very well move it away just as dget_parent() is returning
> to caller.  As the result, you are not guaranteed that ovl_copy_up_one()
> arguments will be anywhere near each other in the tree.  Your locking
> and rechecks might be enough to prevent trouble there, but it's not
> obvious, to put it mildly.

This issue is documented above ovl_copy_up_one().  It's not all that
complicated, I think.

> I'm _very_ sceptical about the idea of delaying copyups that much, BTW;
> there's a damn good reason why all implementations starting with Sun's
> one in 80s did copy directories up as soon as they got looked up.  Saves
> a lot of headache...

Maybe.   If we find not trivially fixable holes in the current
implementation I'm open to that direction.

Delayed copy up has the advantage of allowing pure read-only overlays.

> As for whiteouts... I think we ought to pull these bits of unionmoun
> queue into the common stem and add the missing filesystems to them;
> ext* and ufs are trivial (keep in mind that FFS derivatives, including
> ext*, have d_type in directory entry and type 14 (DT_WHT) is there
> precisely for that purpose).  btrfs also has "dir_type" thing - 8bit
> field...

What about userspace interfaces?  Are we allowed to extend d_type and
st_mode without breaking things?

Thanks,
Miklos
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