Re: [PATCH 3/6] writeback: remove pages_skipped accounting in __block_write_full_page()

Fengguang Wu
2007-08-13T13:51:48Z

On Mon, Aug 13, 2007 at 11:03:21AM +1000, David Chinner wrote:
> > --- linux-2.6.23-rc2-mm2.orig/fs/buffer.c
> > +++ linux-2.6.23-rc2-mm2/fs/buffer.c
> > @@ -1713,7 +1713,6 @@ done:
> > * The page and buffer_heads can be released at any time from
> > * here on.
> > */
> > - wbc->pages_skipped++; /* We didn't write this page */
> > }
> > return err;
>
> Hmmmm - I suspect XFS is going to need a similar fix as well. I'm moving
> house so I'm not going to get a chance to look at this for a week...

I guess as long as the skipped page no longer has dirty bit set both
as a page flag and a radix tree tag(i.e. has been put to io by someone
else), it is OK to not increase wbc->pages_skipped.

> On Sun, Aug 12, 2007 at 05:11:23PM +0800, Fengguang Wu wrote:
> > Miklos Szeredi <miklos@szeredi.hu> and me identified a writeback bug:
> > Basicly they are
> > - during the dd: ~16M
> > - after 30s: ~4M
> > - after 5s: ~4M
> > - after 5s: ~176M
> >
> > The box has 2G memory.
> >
> > Question 1:
> > How come the 5s delays? I run 4 tests in total, 2 of which have such 5s delays.
>
> pdflush runs every five seconds, so that is indicative of the inode being
> written once for 1024 pages, and then delayed to the next pdflush run 5s later.
> perhaps the inodes aren't moving between the lists exactly the way you
> think they are...

Now I figured out the exact situation. When the scan of s_io finishes
with some small inodes, nr_to_write will be positive, fooling kupdate
to quit prematurely. But in fact the big dirty file is on s_more_io
waiting for more io... The attached patch fixes it.

Fengguang
===

Subject: 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. Bug the big dirty file is 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).

So 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.

Signed-off-by: Fengguang Wu <wfg@mail.ustc.edu.cn>
---
fs/fs-writeback.c | 2 ++
include/linux/writeback.h | 1 +
mm/page-writeback.c | 9 ++++++---
3 files changed, 9 insertions(+), 3 deletions(-)

--- linux-2.6.23-rc2-mm2.orig/fs/fs-writeback.c
+++ linux-2.6.23-rc2-mm2/fs/fs-writeback.c
@@ -560,6 +560,8 @@ int generic_sync_sb_inodes(struct super_
if (wbc->nr_to_write <= 0)
break;
}
+ if (!list_empty(&sb->s_more_io))
+ wbc->more_io = 1;
spin_unlock(&inode_lock);
return ret; /* Leave any unwritten inodes on s_io */
}
--- linux-2.6.23-rc2-mm2.orig/include/linux/writeback.h
+++ linux-2.6.23-rc2-mm2/include/linux/writeback.h
@@ -61,6 +61,7 @@ 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 */

void *fs_private; /* For use by ->writepages() */
};
--- linux-2.6.23-rc2-mm2.orig/mm/page-writeback.c
+++ linux-2.6.23-rc2-mm2/mm/page-writeback.c
@@ -382,6 +382,7 @@ static void background_writeout(unsigned
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;
@@ -389,8 +390,9 @@ static void background_writeout(unsigned
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)
+ congestion_wait(WRITE, HZ/10);
+ else if (!wbc.more_io)
break;
}
}
@@ -455,13 +457,14 @@ 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)
congestion_wait(WRITE, HZ/10);
- else
+ else if (!wbc.more_io)
break; /* All the old data is written */
}
nr_to_write -= MAX_WRITEBACK_PAGES - wbc.nr_to_write;