Re: [fuse-devel] [PATCH 14/14] mm: Account for WRITEBACK_TEMP inbalance_dirty_pages

[Maxim V. Patlasov]

Hi,

04/25/2013 07:49 PM, Miklos Szeredi ÐÐÑÐÑ:
> On Thu, Apr 25, 2013 at 4:29 PM, Maxim V. Patlasov
> <mpatlasov@xxxxxxxxxxxxx> wrote:
> > > diff --git a/mm/page-writeback.c b/mm/page-writeback.c
> > > index 0713bfb..c47bcd4 100644
> > > --- a/mm/page-writeback.c
> > > +++ b/mm/page-writeback.c
> > > @@ -1235,7 +1235,8 @@ static void balance_dirty_pages(struct address_space
> > > *mapping,
> > >                   */
> > >                  nr_reclaimable = global_page_state(NR_FILE_DIRTY) +
> > >
> > > global_page_state(NR_UNSTABLE_NFS);
> > > -               nr_dirty = nr_reclaimable +
> > > global_page_state(NR_WRITEBACK);
> > > +               nr_dirty = nr_reclaimable +
> > > global_page_state(NR_WRITEBACK) +
> > > +                       global_page_state(NR_WRITEBACK_TEMP);
> > >                  global_dirty_limits(&background_thresh, &dirty_thresh);
> >
> > Please drop this patch. As we discussed in LSF/MM, the fix above is correct,
> > but it's not enough: we also need to ensure disregard of NR_WRITEBACK_TEMP
> > when balance_dirty_pages() is called from fuse daemon. I'll send a separate
> > patch-set soon.
> Please elaborate.  From a technical perspective "fuse daemon" is very
> hard to define, so anything that relies on whether something came from
> the fuse daemon or not is conceptually broken.

As Mel Gorman pointed out, fuse daemon diving into balance_dirty_pages 
should not kick flusher judging on NR_WRITEBACK_TEMP. Essentially, all 
we need in balance_dirty_pages is:

    if (I'm not fuse daemon)
        nr_dirty += global_page_state(NR_WRITEBACK_TEMP);

The way how to identify fuse daemon was not thoroughly scrutinized 
during LSF/MM. Firstly, I thought it would be enough to set a 
per-process flag handling fuse device open. But now I understand that 
fuse daemon may be quite a complicated multi-threaded multi-process 
construction. I'm going to add new FUSE_NOTIFY to allow fuse daemon 
decide when it works on behalf of draining writeout-s. Having in mind 
that fuse-lib is multi-threaded, I'm also going to inherit the flag on 
copy_process(). Does it make sense for you?

Also, another patch will put this ad-hoc FUSE_NOTIFY under fusermount 
control. This will prevent malicious unprivileged fuse mounts from 
setting the flag for malicious purposes.

Thanks,
Maxim
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