linux-kernel - Re: [PATCH 1/1] fallocate: create FAN_MODIFY and IN

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Date:	Tue, 7 Oct 2014 20:05:59 +0200
From:	Jan Kara <jack@...e.cz>
To:	Heinrich Schuchardt <xypron.glpk@....de>
Cc:	Jan Kara <jack@...e.cz>, Alexander Viro <viro@...iv.linux.org.uk>,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	Eric Paris <eparis@...isplace.org>,
	John McCutchan <john@...nmccutchan.com>,
	Robert Love <rlove@...ve.org>,
	Michael Kerrisk <mtk.manpages@...il.com>
Subject: Re: [PATCH 1/1] fallocate: create FAN_MODIFY and IN_MODIFY events

On Mon 06-10-14 21:10:25, Heinrich Schuchardt wrote:
> On 06.10.2014 16:12, Jan Kara wrote:
> >On Fri 03-10-14 10:19:30, Heinrich Schuchardt wrote:
> >>The fanotify and the inotify API can used to monitor changes of the file
> >>system.
> >>
> >>System call fallocate modifies files. Hence it should trigger the corresponding
> >>fanotify (FAN_MODIFY) and inotify (IN_MODIFY) events.
> >>
> >>This patch adds the missing call to fsnotify_modify.
> >   Well, there are different fallocate() commands and e.g. pure
> >FALLOC_FL_KEEP_SIZE call will not change any data in the file. I'm not sure
> >how much we care but I wanted to point that out...
> 
> The most interesting case is FALLOC_FL_COLLAPSE_RANGE because this
> value allows to create arbitrary file content from random data.
> Hence I think we really need to create FAN_MODIFY in this case.
> 
> As the fallocate(2) man page teaches:
> After a successful call, subsequent writes into the range specified
> by offset and len are guaranteed not to fail because of lack of disk
> space.
> 
> So calling fallocate(fd, FALLOC_FL_KEEP_SIZE, offset, len) may
> result in different outcomes of a subsequent write depending on the
> values of offset and len.
> 
> Calling fallocate for a region already zeroed will not result in any
> data change.
> 
> I would like to compare fallocate() with write().
> 
> When we call write() we always create a FAN_MODIFY event even in the
> case of overwriting with identical data.
> 
> So event FAN_MODIFY does not provide any guarantee that data was
> actually changed.
> 
> In analogy to write() I suggest to keep the logic for fallocate() as
> trivial as possible:
> If fallocate() succeeds, create IN_MODIFY and FAN_MODIFY events.
  OK, makes sense. You can add:
Reviewed-by: Jan Kara <jack@...e.cz>

								Honza
> >>Signed-off-by: Heinrich Schuchardt <xypron.glpk@....de>
> >>---
> >>  fs/open.c | 5 +++++
> >>  1 file changed, 5 insertions(+)
> >>
> >>diff --git a/fs/open.c b/fs/open.c
> >>index d6fd3ac..03aa8e5 100644
> >>--- a/fs/open.c
> >>+++ b/fs/open.c
> >>@@ -295,6 +295,11 @@ int do_fallocate(struct file *file, int mode, loff_t offset, loff_t len)
> >>
> >>  	sb_start_write(inode->i_sb);
> >>  	ret = file->f_op->fallocate(file, mode, offset, len);
> >>+
> >>+	/* Create inotify and fanotify events. */
> >>+	if (ret == 0)
> >>+		fsnotify_modify(file);
> >>+
> >>  	sb_end_write(inode->i_sb);
> >>  	return ret;
> >>  }
> >>--
> >>2.1.0
> >>
> 
-- 
Jan Kara <jack@...e.cz>
SUSE Labs, CR
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