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

[Fengguang Wu]

On Thu, Mar 01, 2012 at 05:38:37PM +0100, Jan Kara wrote:
> On Thu 01-03-12 20:36:40, Wu Fengguang wrote:
> > > Please have a think about all of this and see if you can demonstrate
> > > how the iput() here is guaranteed safe.
> > 
> > There are already several __iget()/iput() calls inside fs-writeback.c.
> > The existing iput() calls already demonstrate its safety?
> > 
> > Basically the flusher works in this way
> > 
> > - the dirty inode list i_wb_list does not reference count the inode at all
> > 
> > - the flusher thread does something analog to igrab() and set I_SYNC
> >   before going off to writeout the inode
> > 
> > - evict() will wait for completion of I_SYNC
>   Yes, you are right that currently writeback code already holds inode
> references and so it can happen that flusher thread drops the last inode
> reference. But currently that could create problems only if someone waits
> for flusher thread to make progress while effectively blocking e.g.
> truncate from happening. Currently flusher thread handles sync(2) and
> background writeback and filesystems take care to not hold any locks
> blocking IO / truncate while possibly waiting for these.
> 
> But with your addition situation changes significantly - now anyone doing
> allocation can block and do allocation from all sorts of places including
> ones where we hold locks blocking other fs activity. The good news is that
> we use GFP_NOFS in such places. So if GFP_NOFS allocation cannot possibly
> depend on a completion of some writeback work, then I'd still be
> comfortable with dropping inode references from writeback code. But Andrew
> is right this at least needs some arguing...

You seem to miss the point that we don't do wait or page allocations
inside queue_pageout_work(). The final iput() will not block the
random tasks because the latter don't wait for completion of the work.

        random task                     flusher thread

        page allocation
          page reclaim
            queue_pageout_work()
              igrab()

                  ......  after a while  ......

                                        execute pageout work                
                                        iput()
                                        <work completed>

There will be some reclaim_wait()s if the pageout works are not
executed quickly, in which case vmscan will be impacted and slowed
down. However it's not waiting for any specific work to complete, so
there is no chance to form a loop of dependencies leading to deadlocks.

The iput() does have the theoretic possibility to deadlock the flusher
thread itself (but not with the other random tasks). Since the flusher
thread has always been doing iput() w/o running into such bugs, we can
reasonably expect the new iput() to be as safe in practical.

Thanks,
Fengguang
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