Re: [PATCH 5/9] writeback: introduce the pageout work

[Fengguang Wu <fengguang.wu@intel.com>]

On Fri, Mar 09, 2012 at 11:15:46AM +0100, Jan Kara wrote:
> On Sat 03-03-12 21:55:58, Wu Fengguang wrote:
> > On Fri, Mar 02, 2012 at 11:57:00AM -0800, Andrew Morton wrote:
> > > On Fri, 2 Mar 2012 18:39:51 +0800
> > > Fengguang Wu <fengguang.wu@intel.com> wrote:
> > > 
> > > > > And I agree it's unlikely but given enough time and people, I
> > > > > believe someone finds a way to (inadvertedly) trigger this.
> > > > 
> > > > Right. The pageout works could add lots more iput() to the flusher
> > > > and turn some hidden statistical impossible bugs into real ones.
> > > > 
> > > > Fortunately the "flusher deadlocks itself" case is easy to detect and
> > > > prevent as illustrated in another email.
> > > 
> > > It would be a heck of a lot safer and saner to avoid the iput().  We
> > > know how to do this, so why not do it?
> > 
> > My concern about the page lock is, it costs more code and sounds like
> > hacking around something. It seems we (including me) have been trying
> > to shun away from the iput() problem. Since it's unlikely we are to
> > get rid of the already existing iput() calls from the flusher context,
> > why not face the problem, sort it out and use it with confident in new
> > code?
>   We can get rid of it in the current code - see my patch set. And also we
> don't have to introduce new iput() with your patch set... I don't think
> using ->writepage() directly on a locked page would be a good thing because
> filesystems tend to ignore it completely (e.g. ext4 if it needs to do an
> allocation, or btrfs) or are much less efficient than when ->writepages()
> is used.  So I'd prefer going through writeback_single_inode() as the rest
> of flusher thread.

Totally agreed. I was also not feeling good to use ->writepage() on
the locked page. It looks very nice to pin the inode with I_SYNC
rather than igrab or lock_page.

> > Let me try it now. The only scheme iput() can deadlock the flusher is
> > for the iput() path to come back to queue some work and wait for it.
>   Let me stop you right here. You severely underestimate the complexity of
> filesystems :). Take for example ext4. To do truncate you need to start a
> transaction, to start a transaction, you have to have a space in journal.
> To have a space in journal, you may have to wait for any other process to
> finish writing. If that process needs to wait for flusher thread to be able
> to finish writing, you have a deadlock. And there are other implicit
> dependencies like this. And it's similar for other filesystems as well. So
> you really want to make flusher thread as light as possible with the
> dependencies.

Ah OK, please forgive my ignorance. Let's get rid of the existing
iput()s in the flusher thread.

Thanks,
Fengguang

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