use memcpy_mcsafe() for copy_to_iter() (was: Re: [PATCH v3 0/9] Series short description)

[Dan Williams]

Ingo, Thomas, Al, any concerns with this series?

On Thu, May 3, 2018 at 5:06 PM, Dan Williams <dan.j.williams@xxxxxxxxx> 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
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