Re: [RFC] fuse writable mmap design

[Peter Zijlstra]

On Thu, 2007-11-15 at 20:57 +0100, Miklos Szeredi wrote:
> > The next point then, I'd expect your fuse_page_mkwrite() to push
> > writeout of your 32-odd mmap pages instead of poll.
> 
> You're talking about this:
> 
> +	wait_event(fc->writeback_waitq,
> +		   fc->numwrite < FUSE_WRITEBACK_THRESHOLD);
> 
> right?  It's one of the things I need to clean out, there's no point
> in fc->numwrite, which is essentially the same as the BDI_WRITEBACK
> counter.
> 
> OTOH, I'm thinking about adding a per-fs limit (adjustable for
> privileged mounts) of dirty+writeback.
> 
> I'm not sure how hard would it be to add support for this into
> balance_dirty_pages().  So I'm thinking of a parameter in struct
> backing_dev_info that is used to clip the calculated per-bdi threshold
> below this maximum.
> 
> How would that affect the proportions algorithm?  What would happen to
> the unused portion?  Would it adapt to the slowed writeback and
> allocate it to some other writer?

The unused part is gone, I've not yet found a way to re-distribute this
fairly.

[ It's one of my open-problems, I can do a min_ratio per bdi, but not
  yet a max_ratio ]

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