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Subject[PATCH 2/6] writeback: introduce writeback_control.inodes_cleaned
The flusher works on dirty inodes in batches, and may quit prematurely
if the batch of inodes happen to be metadata-only dirtied: in this case
wbc->nr_to_write won't be decreased at all, which stands for "no pages
written" but also mis-interpreted as "no progress".

So introduce writeback_control.inodes_cleaned to count the inodes get
cleaned. A non-zero value means there are some progress on writeback,
in which case more writeback can be tried.

about v1: The initial version was to count successful ->write_inode()
calls. However it leads to busy loops for sync() over NFS, because NFS
ridiculously returns 0 (success) while at the same time redirties the
inode. The NFS case can be trivially fixed, however there may be more
hidden bugs in other filesystems..

Acked-by: Jan Kara <jack@suse.cz>
Acked-by: Mel Gorman <mel@csn.ul.ie>
Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
---
fs/fs-writeback.c | 4 ++++
include/linux/writeback.h | 1 +
2 files changed, 5 insertions(+)
--- linux-next.orig/fs/fs-writeback.c 2011-04-20 11:50:10.000000000 +0800
+++ linux-next/fs/fs-writeback.c 2011-04-20 11:53:35.000000000 +0800
@@ -454,6 +454,7 @@ writeback_single_inode(struct inode *ino
* No need to add it back to the LRU.
*/
list_del_init(&inode->i_wb_list);
+ wbc->inodes_cleaned++;
}
}
inode_sync_complete(inode);
@@ -716,6 +717,7 @@ static long wb_writeback(struct bdi_writ
wbc.more_io = 0;
wbc.nr_to_write = write_chunk;
wbc.pages_skipped = 0;
+ wbc.inodes_cleaned = 0;

trace_wbc_writeback_start(&wbc, wb->bdi);
if (work->sb)
@@ -732,6 +734,8 @@ static long wb_writeback(struct bdi_writ
*/
if (wbc.nr_to_write <= 0)
continue;
+ if (wbc.inodes_cleaned)
+ continue;
/*
* Didn't write everything and we don't have more IO, bail
*/
--- linux-next.orig/include/linux/writeback.h 2011-04-20 10:30:29.000000000 +0800
+++ linux-next/include/linux/writeback.h 2011-04-20 11:53:35.000000000 +0800
@@ -34,6 +34,7 @@ struct writeback_control {
long nr_to_write; /* Write this many pages, and decrement
this for each page written */
long pages_skipped; /* Pages which were not written */
+ long inodes_cleaned; /* # of inodes cleaned */

/*
* For a_ops->writepages(): is start or end are non-zero then this is



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