[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.

1 files changed, 0 insertions, 1 deletions

diff --git a/fs/block_dev.c b/fs/block_dev.c
index ee5d3161ee7b..d024df4bd465 100644
--- a/fs/block_dev.c
+++ b/fs/block_dev.c
@@ -758,7 +758,6 @@ struct address_space_operations def_blk_aops = {
 	.prepare_write	= blkdev_prepare_write,
 	.commit_write	= blkdev_commit_write,
 	.writepages	= generic_writepages,
-	.vm_writeback	= generic_vm_writeback,
 	.direct_IO	= blkdev_direct_IO,
 };