Re: dax pmd fault handler never returns to userspace

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

On Wed, 2015-11-18 at 10:53 -0500, Jeff Moyer 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.

Thanks for the report, I'll take a look.

- Ross