Re: [PATCH 5/7] block: clear drain buffer if draining for write command

[James Bottomley <James.Bottomley@HansenPartnership.com>]

On Sat, 2008-02-09 at 10:40 +0900, Tejun Heo wrote:
> Clear drain buffer before chaining if the command in question is a
> write.
> 
> Signed-off-by: Tejun Heo <htejun@gmail.com>
> ---
>  block/blk-merge.c |    3 +++
>  1 files changed, 3 insertions(+), 0 deletions(-)
> 
> diff --git a/block/blk-merge.c b/block/blk-merge.c
> index d50cfc8..d0b9031 100644
> --- a/block/blk-merge.c
> +++ b/block/blk-merge.c
> @@ -221,6 +221,9 @@ new_segment:
>  	} /* segments in rq */
>  
>  	if (q->dma_drain_size && q->dma_drain_needed(rq)) {
> +		if (rq->cmd_flags & REQ_RW)
> +			memset(q->dma_drain_buffer, 0, q->dma_drain_size);
> +

This is a bit performance impacting, isn't it?  If you're worried about
data security from a read overrun, then you could do a lazy clear on
read completion if the overrun was used (i.e. if there's any residual).

James