[Drbd-dev] [PATCH] drbd: fix discard_zeroes_if_aligned regression

[Lars Ellenberg]

On Sun, Jun 02, 2019 at 12:28:31AM +0000, Eric Wheeler wrote:
> On Tue, 28 May 2019, Lars Ellenberg wrote:
> 
> > On Fri, May 10, 2019 at 05:36:32PM +0000, Eric Wheeler wrote:
> > > Hi Lars,
> > > 
> > > We just tried 4.19.x and this bugs still exists. We applied the patch 
> > > which was originally submitted to this thread and it still applies cleanly 
> > > and seems to work for our use case. You mentioned that you had some older 
> > > code which zeroed out unaligned discard requests (or perhaps it was for a 
> > > different purpose) that you may be able to use to patch this. Could you 
> > > dig those up and see if we can get this solved?
> > > 
> > > It would be nice to be able to use drbd with thin backing volumes from the 
> > > vanilla kernel. If this has already been fixed in something newer than 
> > > 4.19, then please point me to the commit.
> > 
> > I think it was merged upstream in 5.0
> > f31e583aa2c2 drbd: introduce P_ZEROES (REQ_OP_WRITE_ZEROES on the "wire")
> 
> Thanks Lars, I appreciate your patch.  
> 
> Your unaligned zerout code in drbd_issue_discard_or_zero_out() looks 
> great.  I particulary like how you adjusted max_discard_sectors to the 
> granularity, as well as alignment handling.  Well thought out.
> 
> Your commit notes that "for backward compatibility, P_TRIM means zero-out, 
> unless the DRBD_FF_WZEROES feature flag is agreed upon during handshake."
> 
> We test our environment by deploying the newer kernel on one of the DRBD 
> servers and checking for regressions---but this will cause a zero-out on 
> the new server because the old server doesn't yet support DRBD_FF_WZEROES.
> 
> For our purpose, can you think of any reason that it would be unsafe to 
> hack the following into drbd_do_features() so the newer version will not 
> zero-out while we test and get both nodes up to the newer version?

If you "fake" agreed_features to include DRBD_FF_WZEROES,
that may start to send the P_ZEROES command,
which the older peer does not understand yet,
triggering a protocol error and disconnect...

You can always use a newer DRBD module with the older kernel,
until you are prepared to upgrade the kernel...

But you knew that.

> diff --git a/drivers/block/drbd/drbd_receiver.c b/drivers/block/drbd/drbd_receiver.c
> index c7ad88d..76191e6 100644
> --- a/drivers/block/drbd/drbd_receiver.c
> +++ b/drivers/block/drbd/drbd_receiver.c
> @@ -5382,6 +5382,8 @@ static int drbd_do_features(struct drbd_connection *connection)
>  	connection->agreed_pro_version = min_t(int, PRO_VERSION_MAX, p->protocol_max);
>  	connection->agreed_features = PRO_FEATURES & be32_to_cpu(p->feature_flags);
>  
> +	connection->agreed_features |= DRBD_FF_WZEROES;
> +
>  	drbd_info(connection, "Handshake successful: "
>  	     "Agreed network protocol version %d\n", connection->agreed_pro_version);

-- 
: Lars Ellenberg
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