linux-ext4 - Re: Buffer state bits

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Message-ID: <20090826212700.GB691@shareable.org>
Date:	Wed, 26 Aug 2009 22:27:00 +0100
From:	Jamie Lokier <jamie@...reable.org>
To:	Jan Kara <jack@...e.cz>
Cc:	linux-fsdevel@...r.kernel.org, linux-ext4@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: Buffer state bits

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.

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.

-- Jamie
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