Re: [patch 1/3] fix illogical behavior in balance_dirty_pages()

[Miklos Szeredi]

> > > > It also makes a deadlock possible when one filesystem is writing data
> > > > through another, and the balance_dirty_pages() for the lower
> > > > filesystem is stalling the writeback for the upper filesystem's
> > > > data (*).
> > > 
> > > I still don't understand this one.  I got lost when belatedly told that
> > > i_mutex had something to do with it.
> > 
> > This deadlock only happens, if there's some bottleneck for writing
> > data to the lower filesystem.   This bottleneck could be 
> > 
> >   - i_mutex, preventing parallel writes to the same inode
> >   - limited number of filesystem threads
> >   - limited request queue length in the upper filesystem
> > 
> > Imagine it this way: balance_dirty_pages() for the lower filesystem is
> > stalling a write() because dirty pages in the upper filesystem are
> > over the limit.  Because there's a bottleneck for writing to the lower
> > filesystem, this is stalling _other_ writes from completing.  So
> > there's no progress in writing back pages from the upper filesystem.
> 
> You mean that someone is stuck in balance_dirty_pages() against the lower
> fs while holding locks which prevent writes into the upper fs from
> succeeding?
> 
> Draw us a picture ;)

Well, not a picture, but a sort of indented call trace:

  [some process, which has a fuse file writably mmaped]
  write fault on upper filesystem
    balance_dirty_pages
      loop...
        submit write requests

  ---------------------------------
  [fuse loopback fs thread 1]
  read request from /dev/fuse
  sys_write
    mutex_lock(i_mutex)
    ...
       copy data to page cache
       balance_dirty_pages
          loop ...
                submit write requests
                write requests completed ...
                dirty still over limit ...
           ... loop forever

  [fuse loopback fs thread 2]
  read request from /dev/fuse
  sys_write
    mute_lock(i_mutex) blocks


The lower filesystem (e.g. ext3) has completed the single write
request that was sent to it, and then it's just looping in
balance_dirty_pages.  The upper (fuse) filesystem has all the dirty
data (over the threshold), either still dirty or waiting in the
request queue as writeback.

Does this help?

Miklos
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