linux-kernel - Re: [PATCH 1/4] vfio: ccw: fix cleanup if cp

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Date:   Wed, 21 Mar 2018 13:49:54 +0100
From:   Halil Pasic <pasic@...ux.vnet.ibm.com>
To:     Dong Jia Shi <bjsdjshi@...ux.vnet.ibm.com>,
        linux-kernel@...r.kernel.org, linux-s390@...r.kernel.org,
        kvm@...r.kernel.org
Cc:     cohuck@...hat.com, borntraeger@...ibm.com,
        pmorel@...ux.vnet.ibm.com
Subject: Re: [PATCH 1/4] vfio: ccw: fix cleanup if cp_prefetch fails



On 03/21/2018 03:08 AM, Dong Jia Shi wrote:
> From: Halil Pasic <pasic@...ux.vnet.ibm.com>
> 
> If the translation of a channel program fails, we may end up attempting
> to clean up (free, unpin) stuff that never got translated (and allocated,
> pinned) in the first place.
> 
> By adjusting the lengths of the chains accordingly (so the element that
> failed, and all subsequent elements are excluded) cleanup activities
> based on false assumptions can be avoided.
> 
> Let's make sure cp_free works properly after cp_prefetch returns with an
> error by setting ch_len to 0 for the ccw chains those are not prefetched.

This sentence used to be:

Let's make sure cp_free works properly after cp_prefetch returns with an
error.

@Dong Jia
I find the 'by setting ch_len to 0 for the ccw chains those are not prefetched'
you added for clarification (I guess) somewhat problematic.
The chain in which the translation failure occurred
+	chain->ch_len = idx;
is shortened so that only the translated elements (ccws) are going to
get cleaned up (on a per element basis) by cp_free. This may or may
not be the first ccw. Subsequent chains are shortened to 0 as there
no translation took place.

So as a result of this change only properly translated ccws are going
to get (re)visited by cp_free as only those may have resources bound
to them which need to be released.

I'm not against improving the commit message. But this ain't
an improvement to me.

> 
> Acked-by: Pierre Morel <pmorel@...ux.vnet.ibm.com>
> Reviewed-by: Dong Jia Shi <bjsdjshi@...ux.vnet.ibm.com>
> Signed-off-by: Halil Pasic <pasic@...ux.vnet.ibm.com>
> Signed-off-by: Dong Jia Shi <bjsdjshi@...ux.vnet.ibm.com>
> ---
>  drivers/s390/cio/vfio_ccw_cp.c | 9 ++++++++-
>  1 file changed, 8 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/s390/cio/vfio_ccw_cp.c b/drivers/s390/cio/vfio_ccw_cp.c
> index d9a2fffd034b..2be114db02f9 100644
> --- a/drivers/s390/cio/vfio_ccw_cp.c
> +++ b/drivers/s390/cio/vfio_ccw_cp.c
> @@ -749,11 +749,18 @@ int cp_prefetch(struct channel_program *cp)
>  		for (idx = 0; idx < len; idx++) {
>  			ret = ccwchain_fetch_one(chain, idx, cp);
>  			if (ret)
> -				return ret;
> +				goto out_err;
>  		}
>  	}
> 
>  	return 0;
> +out_err:
> +	/* Only cleanup the chain elements that where actually translated. */
> +	chain->ch_len = idx;
> +	list_for_each_entry_continue(chain, &cp->ccwchain_list, next) {
> +		chain->ch_len = 0;
> +	}
> +	return ret;
>  }
> 
>  /**
>