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Date:	Fri, 13 Jun 2008 15:35:21 -0700
From:	Mingming <cmm@...ibm.com>
To:	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>
Cc:	Jan Kara <jack@...e.cz>, linux-ext4 <linux-ext4@...r.kernel.org>
Subject: Re: ext4_page_mkwrite and delalloc


On Fri, 2008-06-13 at 08:50 +0530, Aneesh Kumar K.V wrote:
> On Thu, Jun 12, 2008 at 02:00:46PM -0700, Mingming Cao wrote:
> > On Thu, 2008-06-12 at 23:44 +0530, Aneesh Kumar K.V wrote:
> > > Hi,
> > > 
> > > With delalloc we should not do writepage in ext4_page_mkwrite. The idea
> > > with delalloc is to delay the block allocation and make sure we allocate
> > > chunks of blocks together at writepages. So i guess we should update
> > > ext4_page_mkwrite to use write_begin and write_end instead of writepage.
> > 
> > I agree with delayed allocation page_mkwrite is much simplier, just to
> > block reservation to prevent ENOSPC
> > 
> > > Taking i_alloc_sem should protect against parallel truncate and the page
> > > lock should protect against parallel write_begin/write_end.
> > > 
> > > How about the patch below ?
> > > 
> > 
> > Do we plan to support page_mkwrite for non delalloc? the following patch
> > seems suggesting that we only do page_mkwrite with delalloc?
> 
> Yes it is needed for non delalloc also. The primary requirement is for
> lock inversion patches. With lock inversion patches we don't do
> block allocation in writepage
> 
> 
> > 
> > > diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
> > > index cac132b..7f162cc 100644
> > > --- a/fs/ext4/inode.c
> > > +++ b/fs/ext4/inode.c
> > > @@ -3543,18 +3543,6 @@ int ext4_change_inode_journal_flag(struct inode *inode, int val)
> > >  	return err;
> > >  }
> > > 
> > > -static int ext4_bh_prepare_fill(handle_t *handle, struct buffer_head *bh)
> > > -{
> > > -	if (!buffer_mapped(bh)) {
> > > -		/*
> > > -		 * Mark buffer as dirty so that
> > > -		 * block_write_full_page() writes it
> > > -		 */
> > > -		set_buffer_dirty(bh);
> > > -	}
> > > -	return 0;
> > > -}
> > > -
> > >  static int ext4_bh_unmapped(handle_t *handle, struct buffer_head *bh)
> > >  {
> > >  	return !buffer_mapped(bh);
> > > @@ -3596,24 +3584,22 @@ int ext4_page_mkwrite(struct vm_area_struct *vma, struct page *page)
> > >  		if (!walk_page_buffers(NULL, page_buffers(page), 0, len, NULL,
> > >  				       ext4_bh_unmapped))
> > >  			goto out_unlock;
> > > -		/*
> > > -		 * Now mark all the  buffer head dirty so
> > > -		 * that writepage can write it
> > > -		 */
> > > -		walk_page_buffers(NULL, page_buffers(page), 0, len,
> > > -					NULL, ext4_bh_prepare_fill);
> > >  	}
> > >  	/*
> > > -	 * OK, we need to fill the hole... Lock the page and do writepage.
> > > -	 * We can't do write_begin and write_end here because we don't
> > > -	 * have inode_mutex and that allow parallel write_begin, write_end call.
> > > +	 * OK, we need to fill the hole... Lock the page and do write_begin
> > > +	 * write_end. We are not holding inode.i__mutex here. That allow
> > > +	 * parallel write_begin, write_end call.
> > >  	 * (lock_page prevent this from happening on the same page though)
> > >  	 */
> > > -	lock_page(page);
> > > -	wbc.range_start = page_offset(page);
> > > -	wbc.range_end = page_offset(page) + len;
> > > -	ret = mapping->a_ops->writepage(page, &wbc);
> > > -	/* writepage unlocks the page */
> > > +	ret = mapping->a_ops->write_begin(file, mapping, page_offset(page),
> > > +			len, AOP_FLAG_UNINTERRUPTIBLE, &page, NULL);
> > 
> > What is this AOP_FLAG_UNINTERRUPTIBLE flag ? Also shouldn't we test
> > delalloc is enabled?
> > 
> 
> Since we are not doing any real copy here I guess we can say that
> we don't do short write. The flag means that.
> 
> #define AOP_FLAG_UNINTERRUPTIBLE        0x0001 /* will not do a short write */
> 
> > > +	if (ret < 0)
> > > +		goto out_unlock;
> > > +	ret = mapping->a_ops->write_end(file, mapping, page_offset(page),
> > > +			len, len, page, NULL);
> > 
> > I am still puzzled why we need to mark the page dirty in write_end here.
> > Thought only do block reservation in write_begin is enough, we haven't
> > write anything yet...
> 
> 
> The reason is to get the ordered and journaled mode behavior correct.
> We need ensure that the meta-data that got allocated in the write_begin
> get commited in the right order.

I am confused here, I thought this patch is to take advantage of delayed
allocation, so that we could just call the write_begin in mkwrite, there
is only block reservation, but no real block allocation and meta-data
changes?  Thus no need to worry about the ordering?

>  We need add the buffer_heads
> corresponding to the data (page) to the right list in the journal.
> write_end mostly does that.
> 
I probably missed the basic here, I was assuming the patch also based on
the new orderd mode? But with the new ordered mode, this part(using
buffer heads) is replaced with the journal inode list, and with delayed
allocation, the code to ensure the ordering is pushed later at
writepages() time.

> -aneesh
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