linux-kernel - Re: [PATCH v8 6/6] iommu/tegra-smmu: Add pagetable mappings to debugfs

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Date:   Thu, 9 Dec 2021 22:58:15 +0300
From:   Dmitry Osipenko <digetx@...il.com>
To:     Nicolin Chen <nicolinc@...dia.com>
Cc:     thierry.reding@...il.com, joro@...tes.org, will@...nel.org,
        vdumpa@...dia.com, jonathanh@...dia.com,
        linux-tegra@...r.kernel.org, iommu@...ts.linux-foundation.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v8 6/6] iommu/tegra-smmu: Add pagetable mappings to
 debugfs

09.12.2021 22:51, Nicolin Chen пишет:
> On Thu, Dec 09, 2021 at 10:40:42PM +0300, Dmitry Osipenko wrote:
>> External email: Use caution opening links or attachments
>>
>>
>> 09.12.2021 22:32, Nicolin Chen пишет:
>>> On Thu, Dec 09, 2021 at 05:47:18PM +0300, Dmitry Osipenko wrote:
>>>> External email: Use caution opening links or attachments
>>>>
>>>>
>>>> 09.12.2021 10:38, Nicolin Chen пишет:
>>>>> @@ -545,6 +719,15 @@ static void tegra_smmu_detach_as(struct tegra_smmu *smmu,
>>>>>               if (group->swgrp != swgrp)
>>>>>                       continue;
>>>>>               group->as = NULL;
>>>>> +
>>>>> +             if (smmu->debugfs_mappings) {
>>>> Do we really need this check?
>>>>
>>>> Looks like all debugfs_create_dir() usages in this driver are incorrect,
>>>> that function never returns NULL. Please fix this.
>>> debugfs_create_dir returns ERR_PTR on failure. So here should be
>>> to check !IS_ERR. Thanks for pointing it out!
>>>
>>
>> All debugfs functions handle IS_ERR(). GregKH removes all such checks
>> all over the kernel. So the check shouldn't be needed at all, please
>> remove it if it's unneeded or prove that it's needed.
> 
> debugfs_create_file can handle a NULL parent, but not ERR_PTR one,
> and then it puts the new node under the root. So either passing an
> ERR_PTR parent or creating orphan nodes here doesn't sound good...
> 

What makes you say so? Please show the exact source code that will cause
the problem.

The smmu->debugfs_mappings can't ever be NULL and debugfs_create_file
handles the ERR_PTR [1][2].

[1] https://elixir.bootlin.com/linux/latest/source/fs/debugfs/inode.c#L397

[2] https://elixir.bootlin.com/linux/latest/source/fs/debugfs/inode.c#L330