Messages in this thread | | | Subject | Re: [PATCH v4 2/3] iommu/iova: Avoid double-negatives in magazine helpers | From | John Garry <> | Date | Mon, 18 Jan 2021 15:09:47 +0000 |
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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
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