linux-kernel - Re: [PATCH 1/2] ocfs2: Zero the tail cluster when extending past i

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Message-ID: <20100706115827.GG13023@mail.oracle.com>
Date:	Tue, 6 Jul 2010 04:58:28 -0700
From:	Joel Becker <Joel.Becker@...cle.com>
To:	Tao Ma <tao.ma@...cle.com>
Cc:	Dave Chinner <david@...morbit.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	ocfs2-devel@....oracle.com, Dave Chinner <dchinner@...hat.com>,
	Christoph Hellwig <hch@....de>, Mark Fasheh <mfasheh@...e.com>
Subject: Re: [PATCH 1/2] ocfs2: Zero the tail cluster when extending past
 i_size v2

On Tue, Jul 06, 2010 at 03:54:58PM +0800, Tao Ma wrote:
> On 07/06/2010 03:17 PM, Joel Becker wrote:
> >>>+	/* Is there a cluster to zero? */
> >>>+	if (!p_cpos)
> >>>+		goto out;
> >>For unwritten extent, we also need to clear the pages? If yes, the
> >>solution doesn't complete if we have 2 unwritten extent, one
> >>contains i_size while one passes i_size. Here we only clear the
> >>pages for the 1st unwritten extent and leave the 2nd one untouched.
> >
> >	We probably don't need to zero unwritten extents.  We cannot
> >have an extent past i_size, can we?
> we can. AFAICS, ocfs2_change_file_space will allocate unwritten
> extents and does't change i_size.

	Oh, you're right.  We need to walk the entire extent range
between i_size and pos and figure out what needs CoW.  This needs to
happen no matter what.

> >	But you dropped the check for pos_blkno alignment.
> >Unconditionally adding the +1 doesn't seem like a good idea.
> You can add it as you wish.
> I just thought that you add one more extra cluster if pos_blkno
> isn't aligned so as to zero blocks in [pos_cpos_start_block,
> pos_blkno).
> But As I said in the comments, you will soon write pos_blkno(it also
> needs to be CoW since it is within this refcounted extent), so if we
> can CoW it out now, maybe we have a chance to not call
> ocfs2_refcount_cow later.

	I'd much rather let the write handle its own contiguousness.  If
we get lucky, that CoW melds with our CoW.  If we don't get lucky, isn't
it better to have the newly changed area be fully contiguous rather than
have the first extent of it not be and then the remaining extents be?

Joel

-- 

Life's Little Instruction Book #3

	"Watch a sunrise at least once a year."

Joel Becker
Consulting Software Developer
Oracle
E-mail: joel.becker@...cle.com
Phone: (650) 506-8127
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