linux-kernel - Re: [patch] fs: restore nobh

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Message-ID: <20071026165539.GB17254@duck.suse.cz>
Date:	Fri, 26 Oct 2007 18:55:39 +0200
From:	Jan Kara <jack@...e.cz>
To:	Nick Piggin <npiggin@...e.de>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	linux-fsdevel@...r.kernel.org,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [patch] fs: restore nobh

On Fri 26-10-07 00:49:41, Nick Piggin wrote:
> On Thu, Oct 25, 2007 at 09:07:36PM +0200, Jan Kara wrote:
> >   Hi,
> > 
> > > This is overdue, sorry. Got a little complicated, and I've been away from
> > > my filesystem test setup so I didn't want ot send it (lucky, coz I found
> > > a bug after more substantial testing).
> > > 
> > > Anyway, RFC?
> >   Hmm, maybe one comment/question:
> > 
> > > Index: linux-2.6/fs/buffer.c
> > > ===================================================================
> > > --- linux-2.6.orig/fs/buffer.c	2007-10-08 14:09:35.000000000 +1000
> > > +++ linux-2.6/fs/buffer.c	2007-10-08 16:45:28.000000000 +1000
> > ...
> > 
> > > -/*
> > > - * Make sure any changes to nobh_commit_write() are reflected in
> > > - * nobh_truncate_page(), since it doesn't call commit_write().
> > > - */
> > > -int nobh_commit_write(struct file *file, struct page *page,
> > > -		unsigned from, unsigned to)
> > > +int nobh_write_end(struct file *file, struct address_space *mapping,
> > > +			loff_t pos, unsigned len, unsigned copied,
> > > +			struct page *page, void *fsdata)
> > >  {
> > >  	struct inode *inode = page->mapping->host;
> > > -	loff_t pos = ((loff_t)page->index << PAGE_CACHE_SHIFT) + to;
> > > +	struct buffer_head *head = NULL;
> > > +	struct buffer_head *bh;
> > >  
> > > -	if (page_has_buffers(page))
> > > -		return generic_commit_write(file, page, from, to);
> > > +	if (!PageMappedToDisk(page)) {
> > > +		if (unlikely(copied < len) && !page_has_buffers(page))
> > > +			attach_nobh_buffers(page, head);
> > > +		if (page_has_buffers(page))
> > > +			return generic_write_end(file, mapping, pos, len, copied, page, fsdata);
> > > +	}
> >   So we can have a page with attached buffers but we decide to not call
> > generic_write_end()...
> > 
> > >  	SetPageUptodate(page);
> > >  	set_page_dirty(page);
> >   And we just set the page dirty but we leave buffers clean. So cannot
> > subsequent try_to_free_buffers() come, mark page as clean (as buffers
> > are clean) and discard the data?
> 
> Hmm, we might just be OK here because set_page_dirty should dirty the
> buffers. However, I think you have spotted a mistake here and it would be
> better if the generic_write_end test was outside the PageMapedToDisk
> check.
  But set_page_dirty() does not dirty buffers. That is only done by
set_page_dirty_buffers(). Or am I missing something?

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