linux-kernel - Re: [PATCH v5 02/11] powerpc/kernel/iommu: Add new iommu_table_in

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Date:   Tue, 20 Jul 2021 19:41:02 +1000
From:   Alexey Kardashevskiy <aik@...abs.ru>
To:     Leonardo Brás <leobras.c@...il.com>,
        Frederic Barrat <fbarrat@...ux.ibm.com>,
        Michael Ellerman <mpe@...erman.id.au>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        David Gibson <david@...son.dropbear.id.au>,
        kernel test robot <lkp@...el.com>,
        Nicolin Chen <nicoleotsuka@...il.com>
Cc:     linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5 02/11] powerpc/kernel/iommu: Add new
 iommu_table_in_use() helper



On 20/07/2021 15:38, Leonardo Brás wrote:
> Hello Fred, thanks for this feedback!
> 
> Sorry if I miss anything, this snippet was written for v1 over an year
> ago, and I have not taken a look at it ever since.
> 
> On Mon, 2021-07-19 at 15:53 +0200, Frederic Barrat wrote:
>>
>>
>> On 16/07/2021 10:27, Leonardo Bras wrote:
>>> @@ -1099,18 +1105,13 @@ int iommu_take_ownership(struct iommu_table
>>> *tbl)
>>>          for (i = 0; i < tbl->nr_pools; i++)
>>>                  spin_lock_nest_lock(&tbl->pools[i].lock, &tbl-
>>>> large_pool.lock);
>>>    
>>> -       iommu_table_release_pages(tbl);
>>> -
>>> -       if (!bitmap_empty(tbl->it_map, tbl->it_size)) {
>>> +       if (iommu_table_in_use(tbl)) {
>>>                  pr_err("iommu_tce: it_map is not empty");
>>>                  ret = -EBUSY;
>>> -               /* Undo iommu_table_release_pages, i.e. restore
>>> bit#0, etc */
>>> -               iommu_table_reserve_pages(tbl, tbl-
>>>> it_reserved_start,
>>> -                               tbl->it_reserved_end);
>>> -       } else {
>>> -               memset(tbl->it_map, 0xff, sz);
>>>          }
>>>    
>>> +       memset(tbl->it_map, 0xff, sz);
>>> +
>>
>>
>> So if the table is not empty, we fail (EBUSY) but we now also
>> completely
>> overwrite the bitmap. It was in an unexpected state, but we're making
>> it
>> worse. Or am I missing something?
> 
> IIRC there was a reason to do that at the time, but TBH I don't really
> remember it, and by looking at the code right now you seem to be
> correct about this causing trouble.
> 
> I will send a v6 fixing it soon.
> Please review the remaining patches for some issue I may be missing.
> 
> Alexey, any comments on that?


Agree with Fred, this is a bug, EBUSY is not that unexpected :-/ Thanks,


> 
>>
>>     Fred
>>
> 
> Again, thank you for reviewing Fred!
> Best regards,
> Leonardo Bras
> 
> 
> 
> 
> 

-- 
Alexey