> From: Dexuan Cui > Sent: Friday, March 29, 2024 1:23 PM > To: Easwar Hariharan <eahariha@xxxxxxxxxxxxxxxxxxx>; hch@xxxxxx; > m.szyprowski@xxxxxxxxxxx; robin.murphy@xxxxxxx; > zhangpeng362@xxxxxxxxxx; iommu@xxxxxxxxxxxxxxx; > mhklinux@xxxxxxxxxxx > Cc: linux-kernel@xxxxxxxxxxxxxxx; stable@xxxxxxxxxxxxxxx > Subject: RE: [PATCH] swiotlb: Do not set total_used to 0 in > swiotlb_create_debugfs_files() > > > From: Easwar Hariharan <eahariha@xxxxxxxxxxxxxxxxxxx> > > 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
