Re: [PATCH 1/2] ocfs2: Zero the tail cluster when extending pasti_size v2

[Tao Ma]

Joel Becker wrote:
> 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.
>   
Actually we can only have unwritten extents after i_size and it 
shouldn't hurt you in this case.
So do we really need to CoW all the unwritten extents?
All I want to say is that since they are unwritten, they should also 
mean 'zero' for the user space.
So can we just need to skip clearing pages if i_size is in an unwritten 
extent?
>   
> > > 	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?
>   
fair enough.

Regards,
Tao
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