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SubjectRe: Help with implementing some form of barriers in 3.0 kernels.
>>> On 14.09.11 at 10:59, Konrad Rzeszutek Wilk <konrad.wilk@oracle.com> wrote:
> On Tue, Sep 13, 2011 at 11:44:07AM +0100, Jan Beulich wrote:
>> >>> On 07.09.11 at 19:48, Konrad Rzeszutek Wilk <konrad.wilk@oracle.com> wrote:
>> > Hey Christoph,
>> >
>> > I was wondering what you think is the proper way of implementing a
>> > backend to support the 'barrier' type requests? We have this issue were
>> > there are 2.6.36 type guests that still use barriers and we would like
>> > to support them properly. But in 3.0 there are no barriers - hence
>> > the question whether WRITE_fLUSH_FUA would be equal to WRITE_BARRIER?
>> >
>> > Or is there some other things that we should take in consideration?
>> >
>> > Thanks!
>>
>> Below is what Jan Kara came up with for addressing this - what do
>> you think?
>
> It looks like it would do it. I modified it a bit, testing it now.
>
>
> commit 315c0cf1a5174b9aed494d7903133ce9ac76d6f1
> Author: Jan Kara <jack@suse.cz>
> Date: Tue Sep 13 11:44:07 2011 +0100
>
> xen: Add support for old BARRIER requests to xenblk driver
>
> Recent kernels do not support BARRIER operation but only FLUSH
> operation. But
> older xenblk frontends still use the BARRIER operation to achieve data
> integrity requirements. So add support for BARRIER operation into xenblk
> backend so that all guests do not corrupt their filesystem on host
> crash.
>
> Signed-off-by: Jan Kara <jack@suse.cz>
> Signed-off-by: Jan Beulich <JBeulich@suse.com>
> [v1: Added some extra functions, and other cases]

Ah, yes, of course - Jan probably didn't pay attention because in our
variant of the driver these functions never got removed.

> Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
>
> diff --git a/block/elevator.c b/block/elevator.c
> index a3b64bc..b2143a8 100644
> --- a/block/elevator.c
> +++ b/block/elevator.c
> @@ -616,6 +616,7 @@ void elv_drain_elevator(struct request_queue *q)
> q->elevator->elevator_type->elevator_name, q->nr_sorted);
> }
> }
> +EXPORT_SYMBOL(elv_drain_elevator);

Now, if you modify it anyway, how about making this an
EXPORT_SYMBOL_GPL()?

Jan

>
> /*
> * Call with queue lock held, interrupts disabled
> diff --git a/drivers/block/xen-blkback/blkback.c
> b/drivers/block/xen-blkback/blkback.c
> index 9713d5a..1df773c 100644
> --- a/drivers/block/xen-blkback/blkback.c
> +++ b/drivers/block/xen-blkback/blkback.c
> @@ -464,6 +464,11 @@ static void __end_block_io_op(struct pending_req
> *pending_req, int error)
> pr_debug(DRV_PFX "flush diskcache op failed, not supported\n");
> xen_blkbk_flush_diskcache(XBT_NIL, pending_req->blkif->be, 0);
> pending_req->status = BLKIF_RSP_EOPNOTSUPP;
> + } else if ((pending_req->operation == BLKIF_OP_WRITE_BARRIER) &&
> + (error == -EOPNOTSUPP)) {
> + pr_debug(DRV_PFX "write barrier op failed, not supported\n");
> + xen_blkbk_barrier(XBT_NIL, pending_req->blkif->be, 0);
> + pending_req->status = BLKIF_RSP_EOPNOTSUPP;
> } else if (error) {
> pr_debug(DRV_PFX "Buffer not up-to-date at end of operation,"
> " error=%d\n", error);
> @@ -590,6 +595,7 @@ static int dispatch_rw_block_io(struct xen_blkif *blkif,
> struct bio *biolist[BLKIF_MAX_SEGMENTS_PER_REQUEST];
> int i, nbio = 0;
> int operation;
> + bool drain = false;
> struct blk_plug plug;
>
> switch (req->operation) {
> @@ -601,6 +607,8 @@ static int dispatch_rw_block_io(struct xen_blkif *blkif,
> blkif->st_wr_req++;
> operation = WRITE_ODIRECT;
> break;
> + case BLKIF_OP_WRITE_BARRIER:
> + drain = true;
> case BLKIF_OP_FLUSH_DISKCACHE:
> blkif->st_f_req++;
> operation = WRITE_FLUSH;
> @@ -609,7 +617,6 @@ static int dispatch_rw_block_io(struct xen_blkif *blkif,
> blkif->st_ds_req++;
> operation = REQ_DISCARD;
> break;
> - case BLKIF_OP_WRITE_BARRIER:
> default:
> operation = 0; /* make gcc happy */
> goto fail_response;
> @@ -668,6 +675,17 @@ static int dispatch_rw_block_io(struct xen_blkif *blkif,
> }
> }
>
> + if (drain) {
> + struct request_queue *q = bdev_get_queue(preq.bdev);
> + unsigned long flags;
> +
> + /* Emulate the original behavior of write barriers */
> + spin_lock_irqsave(q->queue_lock, flags);
> + elv_drain_elevator(q);
> + __blk_run_queue(q);
> + spin_unlock_irqrestore(q->queue_lock, flags);
> + }
> +
> /*
> * If we have failed at this point, we need to undo the M2P override,
> * set gnttab_set_unmap_op on all of the grant references and perform
> diff --git a/drivers/block/xen-blkback/common.h
> b/drivers/block/xen-blkback/common.h
> index bfb532e..42b0e46 100644
> --- a/drivers/block/xen-blkback/common.h
> +++ b/drivers/block/xen-blkback/common.h
> @@ -228,6 +228,8 @@ int xen_blkif_schedule(void *arg);
>
> int xen_blkbk_flush_diskcache(struct xenbus_transaction xbt,
> struct backend_info *be, int state);
> +int xen_blkbk_barrier(struct xenbus_transaction xbt,
> + struct backend_info *be, int state);
>
> struct xenbus_device *xen_blkbk_xenbus(struct backend_info *be);
>
> diff --git a/drivers/block/xen-blkback/xenbus.c
> b/drivers/block/xen-blkback/xenbus.c
> index 2b3aef0..b477aee 100644
> --- a/drivers/block/xen-blkback/xenbus.c
> +++ b/drivers/block/xen-blkback/xenbus.c
> @@ -421,6 +421,20 @@ int xen_blkbk_flush_diskcache(struct xenbus_transaction
> xbt,
> return err;
> }
>
> +int xen_blkbk_barrier(struct xenbus_transaction xbt,
> + struct backend_info *be, int state)
> +{
> + struct xenbus_device *dev = be->dev;
> + int err;
> +
> + err = xenbus_printf(xbt, dev->nodename, "feature-barrier",
> + "%d", state);
> + if (err)
> + xenbus_dev_fatal(dev, err, "writing feature-barrier");
> +
> + return err;
> +}
> +
> int xen_blkbk_discard(struct xenbus_transaction xbt, struct backend_info
> *be)
> {
> struct xenbus_device *dev = be->dev;
> @@ -706,6 +720,8 @@ again:
> if (err)
> goto abort;
>
> + err = xen_blkbk_barrier(xbt, be, be->blkif->vbd.flush_support);
> +
> err = xen_blkbk_discard(xbt, be);
>
> err = xenbus_printf(xbt, dev->nodename, "sectors", "%llu",




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