linux-kernel - Re: [PATCH] fs: don't remove inotify watchers from alive inode-s (v2)

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Date:	Thu, 25 Sep 2014 10:30:10 +0200
From:	Jan Kara <jack@...e.cz>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Jan Kara <jack@...e.cz>, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Heinrich Schuchardt <xypron.debian@....de>,
	"Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	John McCutchan <john@...nmccutchan.com>,
	Robert Love <rlove@...ve.org>,
	Eric Paris <eparis@...isplace.org>,
	Cyrill Gorcunov <gorcunov@...nvz.org>,
	Pavel Emelyanov <xemul@...allels.com>,
	Andrey Vagin <avagin@...nvz.org>
Subject: Re: [PATCH] fs: don't remove inotify watchers from alive inode-s (v2)

On Wed 24-09-14 13:19:47, Andrew Morton wrote:
> On Wed, 24 Sep 2014 12:51:55 +0200 Jan Kara <jack@...e.cz> wrote:
> 
> >   Hello,
> > 
> >   Andrew, what do you think about the patch below? Al objected that it
> > changes userspace visible behavior some time ago and then he didn't react
> > to our explanations...
> 
> Difficult situation.  There's some really important information missing
> from the changelog:
> 
> - Who cares?  Is there some real application which is hurting from
>   the current situation?  If so, who, what, how and why.  If not, then
>   why change anything?
  I believe Openvz guys hit this in their application but I'll defer to
them for more details.

> - A description of the userspace API change impact.  How did the
>   interface change?  What is the risk of this change causing damage to
>   real applications?
  I believe this was covered in the changelog. Without the patch depending
on the order of unlinks for hardlinked file you sometimes get events:
4       (IN_ATTRIB)
400     (IN_DELETE_SELF)
8000    (IN_IGNORED)

and sometimes you get events:
4       (IN_ATTRIB)
<possibly more events happening for unlinked file>
8       (IN_CLOSE_WRITE)
400     (IN_DELETE_SELF)
8000    (IN_IGNORED)

With the patch you'll always have the second case. So without the patch you
don't receive some events if the file has at least 2 hardlinks and then
gets unlinked. I think the risk that some application relies on *not* getting
those events is pretty low (especially since in the common case of file
without hardlinks you will get all those events).

								Honza
-- 
Jan Kara <jack@...e.cz>
SUSE Labs, CR
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