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    SubjectRe: [PATCH 5/9] writeback: introduce the pageout work
    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 <> 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.


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