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