linux-kernel - Re: [PATCH] Fix queueing work if !bdi_cap_writeback

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Date:	Sun, 16 Sep 2012 23:49:12 +0200
From:	Jan Kara <jack@...e.cz>
To:	OGAWA Hirofumi <hirofumi@...l.parknet.co.jp>
Cc:	Jan Kara <jack@...e.cz>, Fengguang Wu <fengguang.wu@...el.com>,
	viro@...iv.linux.org.uk, hch@....de, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] Fix queueing work if !bdi_cap_writeback_dirty()

On Sat 15-09-12 00:10:53, OGAWA Hirofumi wrote:
> Jan Kara <jack@...e.cz> writes:
> 
> >> If flusher is working, it clears dirty flags of inode. But if those
> >> handers can't flush at the time, we have to do redirty or something to
> >> prevent the reclaim.
> >   Well, if this is your only problem then I'd see better options than just
> > disabling flusher thread. If the inability to write inode is rare, then
> > redirtying seems like a reasonable option (despite I agree it's a bit
> > ugly). If the inability to write is common, then you'll probably have to do
> > the dirty inode tracking yourself in some list and expose inodes to VM when
> > they are ready to be written. Or you handle writing of inodes yourself but
> > leave writing of pages on flusher thread...
> 
> Basically all data can be data-integrity write like data logging, so it
> would be more than common. And ->writepages() will also ignore WBC_SYNC_NONE.
> 
> > Because when you disable flusher thread completely you have to put all the
> > smarts to avoid livelocks, keep fairness among processes, write old data,
> > keep number of dirty pages under control into your filesystem which leads
> > to a lot of duplication.
> 
> I'm not sure what you meant though. What is the difference with ignoring
> WBC_SYNC_NONE?
  When you completely ignore WB_SYNC_NONE writeback, you'll soon drive the
machine close to dirty limits and processes dirtying pages will get
throttled. Because flusher threads won't be able to write pages - they
do WB_SYNC_NONE writeback when we have too many dirty pages - processes
will be throttled until somebody calls sync(1) or someone writes the data
for some other reason... So I suspect things won't really work as you
expect.

								Honza
-- 
Jan Kara <jack@...e.cz>
SUSE Labs, CR
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