linux-ext4 - Re: [PATCH 1/2] ext3: Fix buffer dirtying in data=journal mode

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Message-ID: <20100719184146.GC2456@localhost.localdomain>
Date:	Mon, 19 Jul 2010 14:41:47 -0400
From:	Josef Bacik <josef@...hat.com>
To:	Josef Bacik <josef@...hat.com>
Cc:	Jan Kara <jack@...e.cz>, linux-ext4@...r.kernel.org
Subject: Re: [PATCH 1/2] ext3: Fix buffer dirtying in data=journal mode

On Mon, Jul 19, 2010 at 02:02:22PM -0400, Josef Bacik wrote:
> On Mon, Jun 21, 2010 at 12:42:52PM +0200, Jan Kara wrote:
> > block_prepare_write() can dirty freshly created buffer. This is a problem
> > for data=journal mode because data buffers shouldn't be dirty unless they
> > are undergoing checkpoint. So we have to tweak get_block function for
> > data=journal mode to catch the case when block_prepare_write would dirty
> > the buffer, do the work instead of block_prepare_write, and properly handle
> > dirty buffer as data=journal mode requires it.
> > 
> > It might be cleaner to avoid using block_prepare_write() for data=journal
> > mode writes but that would require us to duplicate most of the function
> > which isn't nice either...
> > 
> > Signed-off-by: Jan Kara <jack@...e.cz>
> > ---
> >  fs/ext3/inode.c |   56 +++++++++++++++++++++++++++++++++++++++++++++++-------
> >  1 files changed, 48 insertions(+), 8 deletions(-)
> > 
> > diff --git a/fs/ext3/inode.c b/fs/ext3/inode.c
> > index ea33bdf..2b61cc4 100644
> > --- a/fs/ext3/inode.c
> > +++ b/fs/ext3/inode.c
> > @@ -993,6 +993,43 @@ out:
> >  	return ret;
> >  }
> >  
> > +static int ext3_journalled_get_block(struct inode *inode, sector_t iblock,
> > +				     struct buffer_head *bh, int create)
> > +{
> > +	handle_t *handle = ext3_journal_current_handle();
> > +	int ret;
> > +
> > +	/* This function should ever be used only for real buffers */
> > +	BUG_ON(!bh->b_page);
> > +
> > +	ret = ext3_get_blocks_handle(handle, inode, iblock, 1, bh, create);
> > +	if (ret > 0) {
> > +		if (buffer_new(bh)) {
> > +			struct page *page = bh->b_page;
> > +
> > +			/*
> > +			 * This is a terrible hack to avoid block_prepare_write
> > +			 * marking our buffer as dirty
> > +			 */
> > +			if (PageUptodate(page)) {
> > +				ret = ext3_journal_get_write_access(handle, bh);
> > +				if (ret < 0)
> > +					goto out;
> > +				unmap_underlying_metadata(bh->b_bdev,
> > +							  bh->b_blocknr);
> > +				clear_buffer_new(bh);
> > +				set_buffer_uptodate(bh);
> > +				ret = ext3_journal_dirty_metadata(handle, bh);
> > +				if (ret < 0)
> > +					goto out;
> > +			}
> > +		}
> 
> Hey Jan,
> 
> It looks like in __block_prepare_write we zero out the end of the page if we're
> not writing to the entire block, but you short-circuit this behavior with this
> get_block.  So it's possible that if we only write to half of the block, the
> last half is going to have whatever stale data was in there before, right?
> Thanks,
> 

Oops, ignore me, nothing changes for the !PageUptodate() case, which is where
the page zero'ing part happens.  Carry on :),

Josef
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