linux-kernel - RE: [PATCH] swiotlb: Do not set total_used to 0 in swiotlb_create_debugfs

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Date: Fri, 29 Mar 2024 20:30:30 +0000
From: Dexuan Cui <decui@...rosoft.com>
To: Easwar Hariharan <eahariha@...ux.microsoft.com>, "hch@....de"
	<hch@....de>, "m.szyprowski@...sung.com" <m.szyprowski@...sung.com>,
	"robin.murphy@....com" <robin.murphy@....com>, "zhangpeng362@...wei.com"
	<zhangpeng362@...wei.com>, "iommu@...ts.linux.dev" <iommu@...ts.linux.dev>,
	"mhklinux@...look.com" <mhklinux@...look.com>
CC: "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"stable@...r.kernel.org" <stable@...r.kernel.org>
Subject: RE: [PATCH] swiotlb: Do not set total_used to 0 in
 swiotlb_create_debugfs_files()

> From: Dexuan Cui
> Sent: Friday, March 29, 2024 1:23 PM
> To: Easwar Hariharan <eahariha@...ux.microsoft.com>; hch@....de;
> m.szyprowski@...sung.com; robin.murphy@....com;
> zhangpeng362@...wei.com; iommu@...ts.linux.dev;
> mhklinux@...look.com
> Cc: linux-kernel@...r.kernel.org; stable@...r.kernel.org
> Subject: RE: [PATCH] swiotlb: Do not set total_used to 0 in
> swiotlb_create_debugfs_files()
> 
> > From: Easwar Hariharan <eahariha@...ux.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