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

[Miklos Szeredi]

> > 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
> 
> This, I assume, is the upper fs
> 
> >   ---------------------------------
> >   [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
> 
> And these, I assume, are handling what you term the lower fs.

All of this is actually the upper, fuse filesystem (maybe except the
filemap/page-writeback parts, which could be argued to belong to the
lower, ext3 filesystem).

Above the line is the kernel part, below the line is the userspace
part.

> > 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?
> 
> yup.
> 
> Interesting problem.  I don't suppose that it'd be appreiated if I were to
> commend the use of O_DIRECT for handling the lower fs ;)

Not really ;)  One of the goals of fuse is to not restrict the
filesystem daemon in any way.  There are zillions of ways to work
around this deadlock, but I don't even want to make it even
theoretically possible.

> Let me think about that a bit, after I've made the latest shitpile people
> have inflicted upon me begin to look like it has a chance of compiling.

OK.

Thanks,
Miklos
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