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Date:	Mon, 16 Jun 2008 23:41:52 +0530
From:	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>
To:	Jan Kara <jack@...e.cz>
Cc:	cmm@...ibm.com, tytso@....edu, sandeen@...hat.com,
	linux-ext4@...r.kernel.org, adilger@....com
Subject: Re: [RFC] ext4: Semantics of delalloc,data=ordered

On Mon, Jun 16, 2008 at 07:20:46PM +0200, Jan Kara wrote:
> > On Mon, Jun 16, 2008 at 05:05:33PM +0200, Jan Kara wrote:
> > >   Hi Aneesh,
> > > 
> > >   First, I'd like to see some short comment on what semantics
> > > delalloc,data=ordered is going to have. At least I can imagine at least
> > > two sensible approaches:
> > >   1) All we guarantee is that user is not going to see uninitialized data.
> > > We send writes to disk (and allocate blocks) whenever it fits our needs
> > > (usually when pdflush finds them).
> > >   2) We guarantee that when transaction commits, your data is on disk -
> > > i.e., we allocate actual blocks on transaction commit.
> > > 
> > >   Both these possibilities have their pros and cons. Most importantly,
> > > 1) gives better disk layout while 2) gives higher consistency
> > > guarantees. Note that with 1), it can under some circumstances happen,
> > > that after a crash you see block 1 and 3 of your 3-block-write on disk,
> > > while block 2 is still a hole. 1) is easy to implement (you mostly did
> > > it below), 2) is harder. I think there should be broader consensus on
> > > what the semantics should be (changed subject to catch more attention
> > > ;).
> > > 
> > >   A few comments to your patch are also below.
> > > 
> > > 								Honza
> > 
> > The way I was looking at ordered mode was, we only guarantee that the
> > meta-data blocks corresponding to the data block allocated get committed
> > only after the data-blocks are written to the disk. As long as we don't
> > allocate blocks corresponding to a page we don't write the  page to
> > disk. This should also speed up the "sync slowness" that lot of people
> > are reporting with ordered mode.
>   I'm not sure if it helps - tons of dirty data have to get to
> transaction at some time even with delayed alloc and at that moment any
> "interactive application" is going to be starved.
> 
> > Can you explain
> > "
> > 1), it can under some circumstances happen, that after a crash you see
> > block 1 and 3 of your 3-block-write on disk, while block 2 is still a hole.
> > "
>   Imagine you have a file with blocks 1 and 3 allocated and block 2 is a
> hole. You write blocks 1-3. Block 2 isn't allocated because of delalloc.
> Now if inode is already in the current transaction's list, during commit
> writes to blocks 1 and 3 will land on disk but write to block 2 will
> happen only after pdflush finds it.

And that should be fine with data=ordered mode right ?. Because since
block 2 is not yet allocated we don't have associated meta-data. So
even if we crash we have meta-data pointing to 1 and 3 and not 2. The
problem is only when we write the meta-data pointing to block 2 and not
block 2 itself ?.


> 
> > > > Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@...ux.vnet.ibm.com>
> > > > ---
> > > >  fs/ext4/inode.c  |  169 +++++++++++++++++++++++++++++++++++++++++++++++++++--
> > > >  fs/jbd2/commit.c |   41 ++++++++++++--
> > > >  2 files changed, 198 insertions(+), 12 deletions(-)
> > > > 
> > > > diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
> > > > index 63355ab..7d87641 100644
> > > > --- a/fs/ext4/inode.c
> > > > +++ b/fs/ext4/inode.c
> > > > @@ -1606,13 +1606,12 @@ static int ext4_bh_unmapped_or_delay(handle_t *handle, struct buffer_head *bh)
> > > >  	return !buffer_mapped(bh) || buffer_delay(bh);
> > > >  }
> > > >  
> > > > -/* FIXME!! only support data=writeback mode */
> > > >  /*
> > > >   * get called vi ext4_da_writepages after taking page lock
> > > >   * We may end up doing block allocation here in case
> > > >   * mpage_da_map_blocks failed to allocate blocks.
> > > >   */
> > > > -static int ext4_da_writepage(struct page *page,
> > > > +static int ext4_da_writeback_writepage(struct page *page,
> > > >  				struct writeback_control *wbc)
> > > >  {
> > > >  	int ret = 0;
> > > > @@ -1660,6 +1659,61 @@ static int ext4_da_writepage(struct page *page,
> > > >  	return ret;
> > > >  }
> > > >  
> > > > +/*
> > > > + * get called vi ext4_da_writepages after taking page lock
> > > > + * We may end up doing block allocation here in case
> > > > + * mpage_da_map_blocks failed to allocate blocks.
> > > > + *
> > > > + * We also get called via journal_submit_inode_data_buffers
> > > > + */
> > > > +static int ext4_da_ordered_writepage(struct page *page,
> > > > +				struct writeback_control *wbc)
> > > > +{
> > > > +	int ret = 0;
> > > > +	loff_t size;
> > > > +	unsigned long len;
> > > > +	handle_t *handle = NULL;
> > > > +	struct buffer_head *page_bufs;
> > > > +	struct inode *inode = page->mapping->host;
> > > > +
> > > > +	handle = ext4_journal_current_handle();
> > > > +	if (!handle) {
> > > > +		/*
> > > > +		 * This can happen when we aren't called via
> > > > +		 * ext4_da_writepages() but directly (shrink_page_list).
> > > > +		 * We cannot easily start a transaction here so we just skip
> > > > +		 * writing the page in case we would have to do so.
> > > > +		 */
> > > > +		size = i_size_read(inode);
> > > > +
> > > > +		page_bufs = page_buffers(page);
> > > > +		if (page->index == size >> PAGE_CACHE_SHIFT)
> > > > +			len = size & ~PAGE_CACHE_MASK;
> > > > +		else
> > > > +			len = PAGE_CACHE_SIZE;
> > > > +
> > > > +		if (walk_page_buffers(NULL, page_bufs, 0,
> > > > +				len, NULL, ext4_bh_unmapped_or_delay)) {
> > > > +			/*
> > > > +			 * We can't do block allocation under
> > > > +			 * page lock without a handle . So redirty
> > > > +			 * the page and return.
> > > > +			 * We may reach here when we do a journal commit
> > > > +			 * via journal_submit_inode_data_buffers.
> > > > +			 * If we don't have mapping block we just ignore
> > > > +			 * them
> > > > +			 *
> > > > +			 */
> > > > +			redirty_page_for_writepage(wbc, page);
> > > > +			unlock_page(page);
> > > > +			return 0;
> > > > +		}
> > > > +	}
> > > > +
> > > > +	ret = block_write_full_page(page, ext4_da_get_block_write, wbc);
> > > > +
> > > > +	return ret;
> > > > +}
> > >   If you're going to use this writepage() implementation from commit
> > > code, you cannot simply do redirty_page_for_writepage() and bail out
> > > when there's an unmapped buffer. You must write out at least mapped
> > > buffers to satisfy ordering guarantees (think of filesystems with
> > > blocksize < page size).
> > 
> > With delalloc is it possible to have a page that have some buffer_heads
> > marked delay ?
>   I thought more about the case where some buffers are mapped and some
> aren't (because there's a hole in the middle of the page)...

With delalloc it won't be a problem. This is what i understood.

We allocate blocks only during writepages. That means we allocate blocks
and write then via block_write_full_page at the same time. Also we add
meta-data to the journal list. We also add inode to the journal list.
We also mark page_writeback on the page. Now when the journal_commit
happens we again walk through the inode and try to write the page. The
page may already have finished writeback by then or it may be in
writeback. In both the case we won't actually be sending any data block to disk
on journal commit. If is it in writeback we would have page_writeback
set and journal commit would wait via filemap_fdatawait.


-aneesh
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