Re: [PATCH] swiotlb: Do not set total_used to 0 in swiotlb_create_debugfs_files()

[Easwar Hariharan <eahariha@linux.microsoft.com>]

On 3/29/2024 1:30 PM, Dexuan Cui wrote:
>> From: Dexuan Cui
>> Sent: Friday, March 29, 2024 1:23 PM
>> To: Easwar Hariharan <eahariha@linux.microsoft.com>; hch@lst.de;
>> m.szyprowski@samsung.com; robin.murphy@arm.com;
>> zhangpeng362@huawei.com; iommu@lists.linux.dev;
>> mhklinux@outlook.com
>> Cc: linux-kernel@vger.kernel.org; stable@vger.kernel.org
>> Subject: RE: [PATCH] swiotlb: Do not set total_used to 0 in
>> swiotlb_create_debugfs_files()
>>
>>> From: Easwar Hariharan <eahariha@linux.microsoft.com>
>>> Sent: Friday, March 29, 2024 12:47 PM
>>> [...]
>>> Sorry, I'm missing a why in this commit message. Can you say what
>> happens
>>> if the total_used and used_hiwater IS blindly set to 0? Is the only effect
>>> the change  in the readout of the swiotlb debugfs files?
>>>
>>> Thanks,
>>> Easwar
>>
>> Right, when the system is not doing any I/O, the readout may
>> return a huge number while it should return 0. This is the only effect.
>>
>> Thanks,
>> Dexuan
> 
> Let me share more details.
> 
> kernel/dma/swiotlb.c uses inc_used_and_hiwater() and dec_used()
> to do the accounting.
> 
> The issue happens this way:
> 
> 1. inc_used_and_hiwater() adds n to total_used.
> 2. swiotlb_create_debugfs_files() sets total_used to 0.
> 3. dec_used() decreases total_used by n, i.e. total_used incorrectly 
> becomes a negative number -n, which is a huge number since
> mem_used() converts the 'long' to 'unsigned long'.
> 
> Thanks,
> Dexuan
> 

Thanks for the detail. I only ask because the patch is marked for stable, and
I was wondering if it meets the criteria. But, as you mentioned off list, two Fixes
tags probably do meet the bar.

Thanks,
Easwar