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@xxxxxxx>
> ---
> 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,
Josef
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