Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
Subject: Re: [PATCH v4 2/3] iommu/iova: Avoid double-negatives in magazine
 helpers
To:     Robin Murphy <robin.murphy@arm.com>,
        Jean-Philippe Brucker <jean-philippe@linaro.org>
CC:     <joro@8bytes.org>, <will@kernel.org>,
        <linux-kernel@vger.kernel.org>, <linuxarm@huawei.com>,
        <iommu@lists.linux-foundation.org>
References: <1607538189-237944-1-git-send-email-john.garry@huawei.com>
 <1607538189-237944-3-git-send-email-john.garry@huawei.com>
 <YAHRKCkcHAEUdRNT@larix.localdomain>
 <69c30e85-4a72-a0e1-1e56-4ffbd0df5aba@huawei.com> <YAVeDOiKBEKZ2Tdq@myrica>
 <cdc77ccd-823d-464b-fe3c-2a9da17bcb61@huawei.com>
 <69614e38-fcc0-4220-e1cd-15de91dd61ef@arm.com>
From:   John Garry <john.garry@huawei.com>
Message-ID: <5fdeed73-190f-3272-03e5-8adc60148521@huawei.com>
Date:   Mon, 18 Jan 2021 15:09:47 +0000
User-Agent: Mozilla/5.0 (Windows NT 10.0; WOW64; rv:68.0) Gecko/20100101
 Thunderbird/68.1.2
MIME-Version: 1.0
In-Reply-To: <69614e38-fcc0-4220-e1cd-15de91dd61ef@arm.com>
Content-Type: text/plain; charset="utf-8"; format=flowed
Content-Language: en-US
Content-Transfer-Encoding: 7bit
Precedence: bulk

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