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 <[email protected]>
> Acked-by: Tejun Heo <[email protected]>
>
> 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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