On 2020-04-24 13:15, Alex Williamson wrote:
On Fri, 24 Apr 2020 12:20:03 -0700 John Hubbard jhubbard@nvidia.com wrote:
On 2020-04-24 11:18, Alex Williamson wrote: ...
Hi John,
I'm seeing a regression bisected back to this commit (3faa52c03f44 mm/gup: track FOLL_PIN pages). I've attached some vfio-pci test code that reproduces this by mmap'ing a page of MMIO space of a device and then tries to map that through the IOMMU, so this should be attempting a gup/pin of a PFNMAP page. Previously this failed gracefully (-EFAULT), but now results in:
Hi Alex,
Thanks for this report, and especially for source code to test it, seeing as how I can't immediately spot the problem just from the crash data so far. I'll get set up and attempt a repro.
Actually this looks like it should be relatively easier than the usual sort of "oops, we leaked a pin_user_pages() or unpin_user_pages() call, good luck finding which one" report that I fear the most. :) This one looks more like a crash that happens directly, when calling into the pin_user_pages_remote() code. Which should be a lot easier to solve...
btw, if you are set up for it, it would be nice to know what source file and line number corresponds to the RIP (get_pfnblock_flags_mask+0x22) below. But if not, no problem, because I've likely got to do the repro in any case.
Hey John,
TBH I'm feeling a lot less confident about this bisect. This was readily reproducible to me on a clean tree a bit ago, but now it eludes me. Let me go back and figure out what's going on before you spend any more time on it. Thanks,
OK. But I'm keeping the repro program! :) It made it quick and easy to set up a vfio test, so it was worth doing in any case.
Anyway, I wanted to double check this just out of paranoia, and so now I have a data point for you: your test program runs and passes for me using today's linux.git kernel, with an NVIDIA GPU as the vfio device, and the kernel log is clean. No hint of any problems.
I traced it a little bit:
# sudo bpftrace -e kprobe:__get_user_pages { @[kstack()] = count(); } Attaching 1 probe... ^C ... @[ __get_user_pages+1 __gup_longterm_locked+176 vaddr_get_pfn+104 vfio_pin_pages_remote+113 vfio_dma_do_map+760 vfio_iommu_type1_ioctl+761 ksys_ioctl+135 __x64_sys_ioctl+22 do_syscall_64+90 entry_SYSCALL_64_after_hwframe+73 ]: 1
...and also verified that it's not actually pinning any pages with that path:
$ grep foll_pin /proc/vmstat nr_foll_pin_acquired 0 nr_foll_pin_released 0
Good luck and let me know if it starts pointing to FOLL_PIN or gup, etc.
thanks,