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Subjectuse memcpy_mcsafe() for copy_to_iter() (was: Re: [PATCH v3 0/9] Series short description)
Ingo, Thomas, Al, any concerns with this series?

On Thu, May 3, 2018 at 5:06 PM, Dan Williams <dan.j.williams@intel.com> wrote:
> Changes since v2 [1]:
>
> * Fix source address increment in mcsafe_handle_tail() (Mika)
>
> * Extend the unit test to inject simulated write faults and validate
> that data is properly transferred.
>
> * Rename MCSAFE_DEBUG to MCSAFE_TEST
>
> [1]: https://lists.01.org/pipermail/linux-nvdimm/2018-May/015583.html
>
> ---
>
> Currently memcpy_mcsafe() is only deployed in the pmem driver when
> reading through a /dev/pmemX block device. However, a filesystem in dax
> mode mounted on a /dev/pmemX block device will bypass the block layer
> and the driver for reads. The filesystem-dax (fsdax) read case uses
> dax_direct_access() and copy_to_iter() to bypass the block layer.
>
> The result of the bypass is that the kernel treats machine checks during
> read as system fatal (reboot) when they could simply be flagged as an
> I/O error, similar to performing reads through the pmem driver. Prevent
> this fatal condition by deploying memcpy_mcsafe() in the fsdax read
> path.
>
> The main differences between this copy_to_user_mcsafe() and
> copy_user_generic_unrolled() are:
>
> * Typical tail/residue handling after a fault retries the copy
> byte-by-byte until the fault happens again. Re-triggering machine
> checks is potentially fatal so the implementation uses source alignment
> and poison alignment assumptions to avoid re-triggering machine
> checks.
>
> * SMAP coordination is handled external to the assembly with
> __uaccess_begin() and __uaccess_end().
>
> * ITER_KVEC and ITER_BVEC can now end prematurely with an error.
>
> The new MCSAFE_TEST facility is proposed as a way to unit test the
> exception handling without requiring an ACPI EINJ capable platform.
>
> ---
>
> Dan Williams (9):
> x86, memcpy_mcsafe: remove loop unrolling
> x86, memcpy_mcsafe: add labels for write fault handling
> x86, memcpy_mcsafe: return bytes remaining
> x86, memcpy_mcsafe: add write-protection-fault handling
> x86, memcpy_mcsafe: define copy_to_iter_mcsafe()
> dax: introduce a ->copy_to_iter dax operation
> dax: report bytes remaining in dax_iomap_actor()
> pmem: switch to copy_to_iter_mcsafe()
> x86, nfit_test: unit test for memcpy_mcsafe()
>
>
> arch/x86/Kconfig | 1
> arch/x86/Kconfig.debug | 3 +
> arch/x86/include/asm/mcsafe_test.h | 75 ++++++++++++++++++++++++
> arch/x86/include/asm/string_64.h | 10 ++-
> arch/x86/include/asm/uaccess_64.h | 14 +++++
> arch/x86/lib/memcpy_64.S | 112 +++++++++++++++++-------------------
> arch/x86/lib/usercopy_64.c | 21 +++++++
> drivers/dax/super.c | 10 +++
> drivers/md/dm-linear.c | 16 +++++
> drivers/md/dm-log-writes.c | 15 +++++
> drivers/md/dm-stripe.c | 21 +++++++
> drivers/md/dm.c | 25 ++++++++
> drivers/nvdimm/claim.c | 3 +
> drivers/nvdimm/pmem.c | 13 +++-
> drivers/s390/block/dcssblk.c | 7 ++
> fs/dax.c | 21 ++++---
> include/linux/dax.h | 5 ++
> include/linux/device-mapper.h | 5 +-
> include/linux/string.h | 4 +
> include/linux/uio.h | 15 +++++
> lib/iov_iter.c | 61 ++++++++++++++++++++
> tools/testing/nvdimm/test/nfit.c | 104 +++++++++++++++++++++++++++++++++
> 22 files changed, 482 insertions(+), 79 deletions(-)
> create mode 100644 arch/x86/include/asm/mcsafe_test.h
> _______________________________________________
> Linux-nvdimm mailing list
> Linux-nvdimm@lists.01.org
> https://lists.01.org/mailman/listinfo/linux-nvdimm

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