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

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Date:   Thu, 22 Mar 2018 11:10:26 +0100
From:   Halil Pasic <pasic@...ux.vnet.ibm.com>
To:     Pierre Morel <pmorel@...ux.vnet.ibm.com>,
        Dong Jia Shi <bjsdjshi@...ux.vnet.ibm.com>
Cc:     linux-kernel@...r.kernel.org, linux-s390@...r.kernel.org,
        kvm@...r.kernel.org, cohuck@...hat.com, borntraeger@...ibm.com
Subject: Re: [PATCH 1/4] vfio: ccw: fix cleanup if cp_prefetch fails



On 03/22/2018 10:37 AM, Pierre Morel wrote:
> On 22/03/2018 03:22, Dong Jia Shi wrote:
>> * Halil Pasic <pasic@...ux.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@...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;
>> I made a mistake. When rewording the message, I missed this part...
>> Sorry for the problem!

np

>>
>>> 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.

Works with me.

> 
> 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.
> "

Sounds good to me.

Halil