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.
Cheers,
Jeff
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