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SubjectRe: [PATCH 00/13] overlay filesystem: request for inclusion (v16)
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On Thu, Mar 14, 2013 at 12:19 AM, Al Viro <viro@zeniv.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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