linux-ext4 - Re: Buffer state bits

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Message-ID: <20090827111403.GA14240@duck.novell.com>
Date:	Thu, 27 Aug 2009 13:14:03 +0200
From:	Jan Kara <jack@...e.cz>
To:	Jamie Lokier <jamie@...reable.org>
Cc:	linux-fsdevel@...r.kernel.org, linux-ext4@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: Buffer state bits

On Wed 26-08-09 22:27:00, Jamie Lokier wrote:
> Jan Kara wrote:
> > BH_Dirty
> > - Ideally, this bit should mean "buffer has data that have to be
> >   written". But it is not quite true. The problem happens when
> >   someone calls set_page_dirty() on the page to which buffers are
> >   attached or similarly when buffers are attached to a dirty
> >   page. Then all buffers attached to the page are marked dirty -
> >   even those that are beyond end of file which obviously should not
> >   be written.
> > 
> >   When buffer is dirty, the page has to be dirty as well (mark
> >   buffer dirty takes care of that). It is not necessarily the other
> >   way around and buffer dirty bit is what ultimately decides whether
> >   the buffer goes to disk or not.
> 
> That last sentence implies page can be dirty while a buffer in the
> page is not dirty.
  Yes, that happens.

> In that case, do buffers beyond the end of file need to be set dirty
> by set_page_dirty()?  If yes, perhaps the text could explain why.
  No, they need not. But it's racy to check i_size in set_page_dirty
because we don't hold i_mutex... I'll add some explanation to the
paragraph.

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