Re: [PATCH 1/4] vfio: ccw: fix cleanup if cp_prefetch fails

Pierre Morel
2018-03-22T09:37:36Z

On 22/03/2018 03:22, Dong Jia Shi wrote:
> * Halil Pasic <pasic@linux.vnet.ibm.com> [2018-03-21 13:49:54 +0100]:
>
>>
>> On 03/21/2018 03:08 AM, Dong Jia Shi wrote:
>>> From: Halil Pasic <pasic@linux.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;
> I made a mistake. When rewording the message, I missed this part...
> Sorry for the problem!
>
>> 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.
> You are right. How about:
> Let's make sure cp_free works properly after cp_prefetch returns with an
> error by setting ch_len of a ccw chain to the number of the translated
> ccws on that chain.

By the way, since you will propose a new version,
you have a long description of the cp_prefetch function in the code.
I think you should modify it according to the changes and describe how and
why the ch_len field of each chain is used and changed by this function.

Something like:

"
For each chain composing the channel program:
On entry ch_len hold the count of CCW to be translated.
On exit ch_len is adjusted to the count of successfully translated CCW.

This allows cp_free to find in ch_len the count of CCW to free in a chain.
"

Could also be inside the commit message.

Pierre

>
>>> Acked-by: Pierre Morel <pmorel@linux.vnet.ibm.com>
>>> Reviewed-by: Dong Jia Shi <bjsdjshi@linux.vnet.ibm.com>
>>> Signed-off-by: Halil Pasic <pasic@linux.vnet.ibm.com>
>>> Signed-off-by: Dong Jia Shi <bjsdjshi@linux.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;
>>> }
>>>
>>> /**
>>>

--
Pierre Morel
Linux/KVM/QEMU in Böblingen - Germany