writeback: introduce writeback_control.more_io to indicate more io

After making dirty a 100M file, the normal behavior is to start the writeback
for all data after 30s delays.  But sometimes the following happens instead:

	- after 30s:    ~4M
	- after 5s:     ~4M
	- after 5s:     all remaining 92M

Some analyze shows that the internal io dispatch queues goes like this:

		s_io            s_more_io
		-------------------------
	1)	100M,1K         0
	2)	1K              96M
	3)	0               96M

1) initial state with a 100M file and a 1K file
2) 4M written, nr_to_write <= 0, so write more
3) 1K written, nr_to_write > 0, no more writes(BUG)

nr_to_write > 0 in (3) fools the upper layer to think that data have all been
written out.  The big dirty file is actually still sitting in s_more_io.  We
cannot simply splice s_more_io back to s_io as soon as s_io becomes empty, and
let the loop in generic_sync_sb_inodes() continue: this may starve newly
expired inodes in s_dirty.  It is also not an option to draw inodes from both
s_more_io and s_dirty, an let the loop go on: this might lead to live locks,
and might also starve other superblocks in sync time(well kupdate may still
starve some superblocks, that's another bug).

We have to return when a full scan of s_io completes.  So nr_to_write > 0 does
not necessarily mean that "all data are written".  This patch introduces a
flag writeback_control.more_io to indicate this situation.  With it the big
dirty file no longer has to wait for the next kupdate invocation 5s later.

Cc: David Chinner <dgc@sgi.com>
Cc: Ken Chen <kenchen@google.com>
Signed-off-by: Fengguang Wu <wfg@mail.ustc.edu.cn>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
index 414df43..71c158a 100644 (file)
--- a/fs/fs-writeback.c
+++ b/fs/fs-writeback.c
@@ -473,6 +473,8 @@ sync_sb_inodes(struct super_block *sb, struct writeback_control *wbc)
                if (wbc->nr_to_write <= 0)
                        break;
        }
+       if (!list_empty(&sb->s_more_io))
+               wbc->more_io = 1;
        return;         /* Leave any unwritten inodes on s_io */
 }
 

diff --git a/include/linux/writeback.h b/include/linux/writeback.h
index 52be879..1200868 100644 (file)
--- a/include/linux/writeback.h
+++ b/include/linux/writeback.h
@@ -62,6 +62,7 @@ struct writeback_control {
        unsigned for_reclaim:1;         /* Invoked from the page allocator */
        unsigned for_writepages:1;      /* This is a writepages() call */
        unsigned range_cyclic:1;        /* range_start is cyclic */
+       unsigned more_io:1;             /* more io to be dispatched */
 };
 
 /*

diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index 3348521..bcdbbf6 100644 (file)
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -553,6 +553,7 @@ static void background_writeout(unsigned long _min_pages)
                        global_page_state(NR_UNSTABLE_NFS) < background_thresh
                                && min_pages <= 0)
                        break;
+               wbc.more_io = 0;
                wbc.encountered_congestion = 0;
                wbc.nr_to_write = MAX_WRITEBACK_PAGES;
                wbc.pages_skipped = 0;
@@ -560,8 +561,9 @@ static void background_writeout(unsigned long _min_pages)
                min_pages -= MAX_WRITEBACK_PAGES - wbc.nr_to_write;
                if (wbc.nr_to_write > 0 || wbc.pages_skipped > 0) {
                        /* Wrote less than expected */
-                       congestion_wait(WRITE, HZ/10);
-                       if (!wbc.encountered_congestion)
+                       if (wbc.encountered_congestion || wbc.more_io)
+                               congestion_wait(WRITE, HZ/10);
+                       else
                                break;
                }
        }
@@ -626,11 +628,12 @@ static void wb_kupdate(unsigned long arg)
                        global_page_state(NR_UNSTABLE_NFS) +
                        (inodes_stat.nr_inodes - inodes_stat.nr_unused);
        while (nr_to_write > 0) {
+               wbc.more_io = 0;
                wbc.encountered_congestion = 0;
                wbc.nr_to_write = MAX_WRITEBACK_PAGES;
                writeback_inodes(&wbc);
                if (wbc.nr_to_write > 0) {
-                       if (wbc.encountered_congestion)
+                       if (wbc.encountered_congestion || wbc.more_io)
                                congestion_wait(WRITE, HZ/10);
                        else
                                break;  /* All the old data is written */