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Date: Wed, 18 Nov 2015 10:00:14 -0700
From: Ross Zwisler <ross.zwisler@...ux.intel.com>
To: Dan Williams <dan.j.williams@...el.com>
Cc: Jeff Moyer <jmoyer@...hat.com>,
linux-fsdevel <linux-fsdevel@...r.kernel.org>,
linux-nvdimm <linux-nvdimm@...1.01.org>,
linux-ext4 <linux-ext4@...r.kernel.org>,
Ross Zwisler <ross.zwisler@...el.com>
Subject: Re: dax pmd fault handler never returns to userspace
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@...hat.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?
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