Re: [block] allow blk_flush_policy to return REQ_FSEQ_DATA independent of *FLUSH

[ajithb.kumar@gmail.com]

Hi,
Could you please provide clarity on the following.
">   Hmmm... yes, this can become a correctness issue if (and only if)
>   blk_queue_flush() is called to change q->flush_flags while requests
>   are in-flight;"
Could you please clarify as to why is it a correctness issue only if blk_queue_flush() is used to change flush_flags when requests are in flight ?  As I understand, XFS does set WRITE_FLUSH_FUA flag in _xfs_buf_ioapply() function irrespective of whether the underlying device supports flush capabilities or not which will flow into blk_insert_flush().  Is my reading of the code correct and is there a general correctness issue here which potentially results in XFS file system corruption in case of an abrupt shutdown independent of q->flush_flags getting changed while request is in flight.

Thanks,
Ajith

On Tuesday, 9 August 2011 20:54:35 UTC+5:30, Jeff Moyer  wrote:
> Hi,
> 
> blk_insert_flush has the following check:
> 
> 	/*
> 	 * If there's data but flush is not necessary, the request can be
> 	 * processed directly without going through flush machinery.  Queue
> 	 * for normal execution.
> 	 */
> 	if ((policy & REQ_FSEQ_DATA) &&
> 	    !(policy & (REQ_FSEQ_PREFLUSH | REQ_FSEQ_POSTFLUSH))) {
> 		list_add_tail(&rq->queuelist, &q->queue_head);
> 		return;
> 	}
> 
> However, blk_flush_policy will not return with policy set to only
> REQ_FSEQ_DATA:
> 
> static unsigned int blk_flush_policy(unsigned int fflags, struct request *rq)
> {
> 	unsigned int policy = 0;
> 
> 	if (fflags & REQ_FLUSH) {
> 		if (rq->cmd_flags & REQ_FLUSH)
> 			policy |= REQ_FSEQ_PREFLUSH;
> 		if (blk_rq_sectors(rq))
> 			policy |= REQ_FSEQ_DATA;
> 		if (!(fflags & REQ_FUA) && (rq->cmd_flags & REQ_FUA))
> 			policy |= REQ_FSEQ_POSTFLUSH;
> 	}
> 	return policy;
> }
> 
> Notice that REQ_FSEQ_DATA is only set if REQ_FLUSH is set.  Fix this
> mismatch by moving the setting of REQ_FSEQ_DATA outside of the REQ_FLUSH
> check.
> 
> Tejun notes:
> 
>   Hmmm... yes, this can become a correctness issue if (and only if)
>   blk_queue_flush() is called to change q->flush_flags while requests
>   are in-flight; otherwise, requests wouldn't reach the function at all.
>   Also, I think it would be a generally good idea to always set
>   FSEQ_DATA if the request has data.
> 
> Cheers,
> Jeff
> 
> Signed-off-by: Jeff Moyer <jmoyer@redhat.com>
> Acked-by: Tejun Heo <tj@kernel.org>
> 
> diff --git a/block/blk-flush.c b/block/blk-flush.c
> index bb21e4c..2d162bd 100644
> --- a/block/blk-flush.c
> +++ b/block/blk-flush.c
> @@ -95,11 +95,12 @@ static unsigned int blk_flush_policy(unsigned int fflags, struct request *rq)
>  {
>  	unsigned int policy = 0;
>  
> +	if (blk_rq_sectors(rq))
> +		policy |= REQ_FSEQ_DATA;
> +
>  	if (fflags & REQ_FLUSH) {
>  		if (rq->cmd_flags & REQ_FLUSH)
>  			policy |= REQ_FSEQ_PREFLUSH;
> -		if (blk_rq_sectors(rq))
> -			policy |= REQ_FSEQ_DATA;
>  		if (!(fflags & REQ_FUA) && (rq->cmd_flags & REQ_FUA))
>  			policy |= REQ_FSEQ_POSTFLUSH;
>  	}
> --
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