linux-kernel - Re: [PATCH v4 2/3] iommu/iova: Avoid double-negatives in magazine helpers

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Date:   Mon, 18 Jan 2021 15:09:47 +0000
From:   John Garry <john.garry@...wei.com>
To:     Robin Murphy <robin.murphy@....com>,
        Jean-Philippe Brucker <jean-philippe@...aro.org>
CC:     <joro@...tes.org>, <will@...nel.org>,
        <linux-kernel@...r.kernel.org>, <linuxarm@...wei.com>,
        <iommu@...ts.linux-foundation.org>
Subject: Re: [PATCH v4 2/3] iommu/iova: Avoid double-negatives in magazine
 helpers

On 18/01/2021 12:59, Robin Murphy wrote:
>>>>> for cpu_rcaches too, and get a similar abort at runtime.
>>>> It's not specifically that we expect them (allocation failures for the
>>>> loaded magazine), rather we should make safe against it.
>>>>
>>>> So could you be more specific in your concern for the cpu_rcache 
>>>> failure?
>>>> cpu_rcache magazine assignment comes from this logic.
>>> If this fails:
>>>
>>> drivers/iommu/iova.c:847: rcache->cpu_rcaches = 
>>> __alloc_percpu(sizeof(*cpu_rcache), cache_line_size());
>>>
>>> then we'll get an Oops in __iova_rcache_get(). So if we're making the
>>> module safer against magazine allocation failure, shouldn't we also
>>> protect against cpu_rcaches allocation failure?
>>
>> Ah, gotcha. So we have the WARN there, but that's not much use as this 
>> would still crash, as you say.
>>
>> So maybe we can embed the cpu rcaches in iova_domain struct, to avoid 
>> the separate (failable) cpu rcache allocation.
> 
> Is that even possible? The size of percpu data isn't known at compile 
> time, so at best it would add ugly runtime complexity to any allocation 
> of a struct iova_domain by itself, but worse than that it means that 
> embedding iova_domain in any other structure becomes completely broken, no?

Ah, now I see that it's not possible. I was thinking of using 
DEFINE_PER_CPU(), but it's not permitted.

So even though this patch saves us from cpu_rcache->loaded / ->prev == 
NULL, I still prefer not to add explicit checks for cpu_rcache == NULL 
in the IOVA alloc/free paths, and would rather pass an error back in 
init_iova_rcaches(), but adding code for tidy-up looks messy.

Thanks,
John