[PATCH 3/4] writeback: pay attention to wbc->nr_to_write in write_cache_pages

[tytso at mit.edu]

On Fri, Apr 30, 2010 at 12:43:29PM -0700, Andrew Morton wrote:
> 
> Maybe that fs shouldn't be calling write_cache_pages() at all.  After
> all, write_cache_pages() is a wrapper which emits a sequence of calls
> to ->writepage(), and ->writepage() writes a page.

On my todo list is to fix ext4 to not call write_cache_pages() at all.
We are seriously abusing that function ATM, since we're not actually
writing the pages when we call write_cache_pages().  I won't go into
what we're doing, because it's too embarassing, but suffice it to say
that we end up calling pagevec_lookup() or pagevec_lookup_tag()
*four*, count them *four* times while trying to do writeback.

I have a simple patch that gives ext4 our own copy of
write_cache_pages(), and then simplifies it a lot, and fixes a bunch
of problems, but then I discarded it in favor of fundamentally redoing
how we do writeback at all, but it's going to take a while to get
things completely right.  But I am working to try to fix this.

If it would help, I can ressurect the "fork write_cache_pages() and
simplify" patch, so ext4 isn't dependent on the mm/page-writeback.c's
write_cache_pages(), if there is an immediate, short-term need to fix
that function.

						- Ted