I was examining an issue where a user process utilizing vfio is hitting the RLIMIT_MEMLOCK limit during a ioctl(VFIO_IOMMU_MAP_DMA) call. The amount of memory, though, should have been well below the memlock limit. The test maps the same address range to multiple devices. Each time the same address range is mapped to another device, the lock count is increasing, creating a multiplier on the memory lock accounting, which was unexpected to me. Another strange thing, the /proc/PID/status shows VmLck is indeed increasing toward the limit, but /proc/PID/smaps shows that nothing has been locked. The mlock() syscall doesn't doubly account for previously locked ranges when asked to lock them again, so I was initially expecting the same behavior with vfio since they subscribe to the same limit. So a few initial questions: Is there a reason vfio is doubly accounting for the locked pages for each device they're mapped to? Is the discrepency on how much memory is locked depending on which source I consult expected?
