Subject: Re: [PATCH RFC] ext3 data=guarded v5
From: Chris Mason <chris.mason@oracle.com>
To: Jan Kara <jack@suse.cz>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
       "Theodore Ts'o" <tytso@mit.edu>,
       Linux Kernel Developers List <linux-kernel@vger.kernel.org>,
       Ext4 Developers List <linux-ext4@vger.kernel.org>,
       Mike Galbraith <efault@gmx.de>
In-Reply-To: <20090429200412.GA27924@duck.suse.cz>
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	 <20090429200412.GA27924@duck.suse.cz>
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Date: Wed, 29 Apr 2009 16:37:01 -0400
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On Wed, 2009-04-29 at 22:04 +0200, Jan Kara wrote:

> > What we don't want to do is have a call to write() over existing blocks
> > in the file add new things to the data=ordered list.  I don't see how we
> > can avoid that without datanew.
>   Yes, what I suggest would do exactly that:
> In ordered_writepage() in the beginning we do:
>   page_bufs = page_buffers(page);
>   if (!walk_page_buffers(NULL, page_bufs, 0, PAGE_CACHE_SIZE,
>                          NULL, buffer_unmapped)) {
>   	return block_write_full_page(page, NULL, wbc);
>   }
> So we only get to starting a transaction and file some buffers if some buffer
> in the page is unmapped. Write() maps / allocates all buffers in write_begin()
> so they are never added to ordered lists in writepage().

Right, writepage doesn't really need datanew.

>  We rely on write_end
> to do it. So the only case where not all buffers in the page are mapped is
> when we have to allocate in writepage() (mmaped write) or the two cases I
> describe above.

But I still think write_end does need datanew.  That's where 99% of the
ordered buffers are going to come from when we overwrite the contents of
an existing file.

-chris


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