[PATCH] Remove mapping->vm_writeback
The vm_writeback address_space operation was designed to provide the VM
with a "clustered writeout" capability. It allowed the filesystem to
perform more intelligent writearound decisions when the VM was trying
to clean a particular page.
I can't say I ever saw any real benefit from this - not much writeout
actually happens on that path - quite a lot of work has gone into
minimising it actually.
The default ->vm_writeback a_op which I provided wrote back the pages
in ->dirty_pages order. But there is one scenario in which this causes
problems - writing a single 4G file with mem=4G. We end up with all of
ZONE_NORMAL full of dirty pages, but all writeback effort is against
highmem pages. (Because there is about 1.5G of dirty memory total).
Net effect: the machine stalls ZONE_NORMAL allocation attempts until
the ->dirty_pages writeback advances onto ZONE_NORMAL pages.
This can be fixed most sweetly with additional radix-tree
infrastructure which will be quite complex. Later.
So this patch dumps it all, and goes back to using writepage
against individual pages as they come off the LRU.