Re: [PATCH 2/6] writeback: reduce calls to global_page_state in balance_dirty_pages()

[Wu Fengguang <fengguang.wu@intel.com>]

> > The global threshold check is added in place of clip_bdi_dirty_limit()
> > for safety and not intended as a behavior change. If ever leading to
> > big behavior change and regression, that it would be indicating some
> > too permissive per-bdi threshold calculation.
> > 
> > Did you see the global dirty threshold get exceeded when writing to 2+
> > devices? Occasional small exceeding should be OK though. I tried the
> > following debug patch and see no warnings when doing two concurrent cp
> > over local disk and NFS.
>   Oops, sorry. I've misread the code. You're right. There shouldn't be a big
> change in the behavior.

It does indicate a missing point in the changelog. The paragraph is
updated to:

        We now set and clear dirty_exceeded not only based on bdi dirty limits,
        but also on the global dirty limit. The global limit check is added in
        place of clip_bdi_dirty_limit() for safety and not intended as a
        behavior change. The bdi limits should be tight enough to keep all dirty
        pages under the global limit at most time; occasional small exceeding
        should be OK though. The change makes the logic more obvious: the global
        limit is the ultimate goal and shall be always imposed.

Thanks,
Fengguang