linux-kernel - Re: [PATCH 03/17] writeback: introduce writeback_control.inodes

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Date:	Mon, 16 May 2011 19:14:29 +0800
From:	Wu Fengguang <fengguang.wu@...el.com>
To:	Christoph Hellwig <hch@...radead.org>
Cc:	Dave Chinner <david@...morbit.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Jan Kara <jack@...e.cz>, Mel Gorman <mel@....ul.ie>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 03/17] writeback: introduce
 writeback_control.inodes_cleaned

On Mon, May 16, 2011 at 06:40:16PM +0800, Christoph Hellwig wrote:
> On Mon, May 16, 2011 at 09:50:21AM +1000, Dave Chinner wrote:
> > > - nr_to_write has always been "# of pages written" and writeback_sb_inodes()
> > >   is actually making use of it to do page accounting in work->nr_pages.
> > 
> > Do we really care whether it's inodes or pages that are written? As
> > far as i can tell it doesn't, because writing inodes generally
> > requires more IO and so needs to be limited anyway.
> 
> We do care, but the current infrastructure already is bad enough to
> not make it work.  E.g. when calling from balance_dirty_pages we
> couldn't care less if the inode is written back, we just want pages
> on stable storage, similar for wakeups from the VM code.  Sooner or
> later there's no way around splitting page and inode writeback
> completely.

Agreed.

> > So put the accounting in the post-write code in
> > writeback_single_inode() where we already check if the inode is
> > still dirty or not.  Splitting per-inode post-write processing
> > between writeback_single_inode and the higher level code is cludgy -
> > I'd much prefer it done in only one place.
> 
> I'd tend to agree.  Especially as cleaner separation was the main
> goal for getting rid of the writeback_control overload in the beginning.

I wonder if this is what you want, which puts page and inode
accounting together:

                writeback_single_inode(inode, wb, &wbc);
               
                work->nr_pages -= write_chunk - wbc.nr_to_write;
                wrote += write_chunk - wbc.nr_to_write;
+               if (!(inode->i_state & I_DIRTY))
+                       wrote++;
+
                if (wbc.pages_skipped) {
                        /*
                         * writeback is not making progress due to locked
                         * buffers.  Skip this inode for now.
                         */
                        redirty_tail(inode, wb);
                }

Thanks,
Fengguang
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