linux-kernel - [PATCH v2 0/9] use memcpy_mcsafe() for copy_to

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Date:   Wed, 02 May 2018 21:58:35 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     linux-nvdimm@...ts.01.org
Cc:     Tony Luck <tony.luck@...el.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Borislav Petkov <bp@...en8.de>,
        Mike Snitzer <snitzer@...hat.com>, x86@...nel.org,
        Thomas Gleixner <tglx@...utronix.de>,
        Christoph Hellwig <hch@....de>,
        Ross Zwisler <ross.zwisler@...ux.intel.com>,
        Andy Lutomirski <luto@...capital.net>,
        Ingo Molnar <mingo@...hat.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org, tony.luck@...el.com,
        linux-fsdevel@...r.kernel.org
Subject: [PATCH v2 0/9] use memcpy_mcsafe() for copy_to_iter()

Changes since v1 [1]:

* Remove the loop unrolling in the assembly implementation since it
  significantly complicates the exception handling (Linus)

* Introduce a ->copy_to_iter() dax operation for symmetry with the
  existing ->copy_from_iter() operation to allow platform /
  device-specific implementations.

[1]: https://lists.01.org/pipermail/linux-nvdimm/2018-May/015548.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_DEBUG 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_debug.h |   50 ++++++++++++++++
 arch/x86/include/asm/string_64.h    |   10 ++-
 arch/x86/include/asm/uaccess_64.h   |   14 ++++
 arch/x86/lib/memcpy_64.S            |  109 ++++++++++++++++-------------------
 arch/x86/lib/usercopy_64.c          |   17 +++++
 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    |   48 +++++++++++++++
 22 files changed, 394 insertions(+), 79 deletions(-)
 create mode 100644 arch/x86/include/asm/mcsafe_debug.h