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Message-ID: <555fa5aa-a575-d783-dc97-79f63dcf2f57@arm.com>
Date: Tue, 6 Sep 2022 14:37:18 +0100
From: Robin Murphy <robin.murphy@....com>
To: John Garry <john.garry@...wei.com>,
Ethan Zhao <haifeng.zhao@...ux.intel.com>, joro@...tes.org,
will@...nel.org
Cc: iommu@...ts.linux.dev, linux-kernel@...r.kernel.org,
linuxarm@...wei.com
Subject: Re: [PATCH v2 1/2] iova: Remove some magazine pointer NULL checks
On 2022-09-06 11:50, John Garry wrote:
> On 06/09/2022 10:28, Ethan Zhao wrote:
>
> Hi Ethan,
>
>>> Signed-off-by: John Garry <john.garry@...wei.com>
>>> Reviewed-by: Robin Murphy <robin.murphy@....com>
>>> ---
>>> drivers/iommu/iova.c | 7 ++-----
>>> 1 file changed, 2 insertions(+), 5 deletions(-)
>>>
>>> diff --git a/drivers/iommu/iova.c b/drivers/iommu/iova.c
>>> index 47d1983dfa2a..580fdf669922 100644
>>> --- a/drivers/iommu/iova.c
>>> +++ b/drivers/iommu/iova.c
>>> @@ -661,9 +661,6 @@ iova_magazine_free_pfns(struct iova_magazine
>>> *mag, struct iova_domain *iovad)
>>> unsigned long flags;
>>> int i;
>>> - if (!mag)
>>> - return;
>>> -
>>
>> iommu_probe_device
>> ops->probe_finalize(dev);
>> intel_iommu_probe_finalize
>> iommu_setup_dma_ops
>> iommu_dma_init_domain(domain, dma_base, dma_limit, dev)
>> iova_domain_init_rcaches
>> {
>> ...
>> cpu_rcache->loaded = iova_magazine_alloc(GFP_KERNEL);
>> cpu_rcache->prev = iova_magazine_alloc(GFP_KERNEL);
>> if (!cpu_rcache->loaded || !cpu_rcache->prev) {
>> ret = -ENOMEM;
>> goto out_err;
>>
>> Do you mean iova_magazine_alloc() is impossible to fail ?
>
> No, iova_magazine_alloc() may fail and return NULL. But if it does then
> we set iovad rcache pointer = NULL in the error path and don't use the
> rcache.
>
> However we have a !iovad->rcache check on the "fast" alloc but not
> "insert". I need to check why that is again.
Right, if you find a good reason to respin the patch then perhaps also
tweaking the commit message to clarify that it's impossible to have a
NULL rcache *at any point where those checks are made* might avoid all
possible doubt, however I'd hope that it's clear enough that the
transient case while iova_domain_init_rcaches() is in the process of
failing really doesn't need consideration in its own right.
I guess the check in iova_rcache_get() was maybe with the intent of
allowing alloc_iova_fast() to seamlessly fall back to standard
allocation, so an API user can treat iova_domain_init_rcaches() failure
as non-fatal? That makes a fair amount of sense, but does mean that
we're missing the equivalent in iova_rcache_insert() for it to actually
work. Or we just remove it and tighten up the documentation to say
that's not valid - I would like a way to make rcaches optional in
iommu-dma for systems where they're a pointless waste of memory, but we
can always revisit this when we get there.
Cheers,
Robin.
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