Re: dax pmd fault handler never returns to userspace

[Ross Zwisler <ross.zwisler@linux.intel.com>]

On Wed, Nov 18, 2015 at 08:52:59AM -0800, Dan Williams wrote:
> On Wed, Nov 18, 2015 at 7:53 AM, Jeff Moyer <jmoyer@redhat.com> wrote:
> > Hi,
> >
> > When running the nvml library's test suite against an ext4 file system
> > mounted with -o dax, I ran into an issue where many of the tests would
> > simply timeout.  The problem appears to be that the pmd fault handler
> > never returns to userspace (the application is doing a memcpy of 512
> > bytes into pmem).  Here's the 'perf report -g' output:
> >
> > -   88.30%     0.01%  blk_non_zero.st  libc-2.17.so                  [.] __memmove_ssse3_back
> >    - 88.30% __memmove_ssse3_back
> >       - 66.63% page_fault
> >          - 66.47% do_page_fault
> >             - 66.16% __do_page_fault
> >                - 63.38% handle_mm_fault
> >                   - 61.15% ext4_dax_pmd_fault
> >                      - 45.04% __dax_pmd_fault
> >                         - 37.05% vmf_insert_pfn_pmd
> >                            - track_pfn_insert
> >                               - 35.58% lookup_memtype
> >                                  - 33.80% pat_pagerange_is_ram
> >                                     - 33.40% walk_system_ram_range
> >                                        - 31.63% find_next_iomem_res
> >                                             21.78% strcmp
> >
> > And here's 'perf top':
> >
> > Samples: 2M of event 'cycles:pp', Event count (approx.): 56080150519
> > Overhead  Shared Object            Symbol
> >   22.55%  [kernel]                 [k] strcmp
> >   20.33%  [unknown]                [k] 0x00007f9f549ef3f3
> >   10.01%  [kernel]                 [k] native_irq_return_iret
> >    9.54%  [kernel]                 [k] find_next_iomem_res
> >    3.00%  [jbd2]                   [k] start_this_handle
> >
> > This is easily reproduced by doing the following:
> >
> > git clone https://github.com/pmem/nvml.git
> > cd nvml
> > make
> > make test
> > cd src/test/blk_non_zero
> > ./blk_non_zero.static-nondebug 512 /path/to/ext4/dax/fs/testfile1 c 1073741824 w:0
> >
> > I also ran the test suite against xfs, and the problem is not present
> > there.  However, I did not verify that the xfs tests were getting pmd
> > faults.
> >
> > I'm happy to help diagnose the problem further, if necessary.
> 
> Sysrq-t or sysrq-w dump?  Also do you have the locking fix from Yigal?
> 
> https://lists.01.org/pipermail/linux-nvdimm/2015-November/002842.html

I was able to reproduce the issue in my setup with v4.3, and the patch from
Yigal seems to solve it.  Jeff, can you confirm?