linux-kernel - Re: [PATCH v2 3/3] vfs: Use per-cpu list for superblock's inode list

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Date:	Mon, 22 Feb 2016 23:18:55 +0100
From:	Jan Kara <jack@...e.cz>
To:	Dave Chinner <david@...morbit.com>
Cc:	Jan Kara <jack@...e.cz>, Peter Zijlstra <peterz@...radead.org>,
	Waiman Long <Waiman.Long@....com>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Jan Kara <jack@...e.com>,
	Jeff Layton <jlayton@...chiereds.net>,
	"J. Bruce Fields" <bfields@...ldses.org>,
	Tejun Heo <tj@...nel.org>,
	Christoph Lameter <cl@...ux-foundation.org>,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	Ingo Molnar <mingo@...hat.com>,
	Andi Kleen <andi@...stfloor.org>,
	Dave Chinner <dchinner@...hat.com>,
	Scott J Norton <scott.norton@...com>,
	Douglas Hatch <doug.hatch@...com>
Subject: Re: [PATCH v2 3/3] vfs: Use per-cpu list for superblock's inode list

On Tue 23-02-16 08:08:14, Dave Chinner wrote:
> On Mon, Feb 22, 2016 at 02:04:35PM +0100, Jan Kara wrote:
> > On Mon 22-02-16 13:12:22, Peter Zijlstra wrote:
> > > On Mon, Feb 22, 2016 at 12:54:35PM +0100, Jan Kara wrote:
> > > > > Also, I think fsnotify_unmount_inodes() (as per mainline) is missing a
> > > > > final iput(need_iput) at the very end, but I could be mistaken, that
> > > > > code hurts my brain.
> > > > 
> > > > I think the code is actually correct since need_iput contains "inode
> > > > further in the list than the current inode". Thus we will always go though
> > > > another iteration of the loop which will drop the reference. And inode
> > > > cannot change state to I_FREEING or I_WILL_FREE because we hold inode
> > > > reference. But it is subtle as hell so I agree that code needs rewrite.
> > > 
> > > So while talking to dchinner, he doubted fsnotify will actually remove
> > > inodes from the sb-list, but wasn't sure and too tired to check now.
> > > 
> > > (I got lost in the fsnotify code real quick and gave up, for I was
> > > mostly trying to make a point that we don't need the CPP magic and can
> > > do with 'readable' code).
> > > 
> > > If it doesn't, it doesn't need to do this extra special magic dance and
> > > can use the 'normal' iterator pattern used in all the other functions,
> > > greatly reducing complexity.
> > 
> > Yeah, that would be nice. But fsnotify code needs to iterate over all
> > inodes, drop sb_list_lock and do some fsnotify magic with the inode which
> > is not substantial for our discussion. Now that fsnotify magic may actually
> > drop all the remaining inode references so once we drop our reference
> > pinning the inode, it can just disappear. We don't want to restart the scan
> > for each inode we have to process so that is the reason why we play ugly
> > tricks with pinning the next inode in the list.
> > 
> > But I agree it should be possible to just use list_for_each_entry() instead
> > of list_for_each_entry_safe() and keep current inode pinned till the next
> > iteration to make it stick in the sb->s_inodes list. That would make the
> > iteration more standard. Lightly tested patch attached.
> 
> That's exactly what I was thinking. Patch looks ok from aquick
> reading of it, but I haven't I've got anything here to test it
> at all. Perhaps we need so xfstests coverage of this code....

I've tested it by adding watches to some files and then unmounting the
filesystem. That should give basic testing to the code. There's some
reasonable inotify coverage (including unmount events) in LTP but most of
the testing happens in tmpdir so it is not particularly useful for
stressing this code.

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