Re: [PATCH v7 00/19] gfs2: Fix mmap + page fault deadlocks

On Fri, Aug 27, 2021 at 5:51 PM Andreas Gruenbacher [off-list ref] wrote:

Hi all,

here's another update on top of v5.14-rc7.  Changes:

 * Some of the patch descriptions have been improved.

 * Patch "gfs2: Eliminate ip->i_gh" has been moved further to the front.

At this point, I'm not aware of anything that still needs fixing,

Hi, thanks for doing this.

In btrfs we also have a deadlock (after the conversion to use iomap
for direct IO) triggered by your recent test case for fstests,
generic/647 [1].
Even though we can fix it in btrfs without touching iomap, iov_iter,
etc, it would be too complex for such a rare and exotic case (a user
passing a buffer for a direct IO read/write that is memory mapped to
the same file range of the operation is very uncommon at least). But
this patchset would make the fix much simpler and cleaner.

One thing I noticed is that, for direct IO reads, despite setting the
->nofault attribute of the iov_iter to true, we can still get page
faults while in the iomap code.
This happens when reading from holes and unwritten/prealloc extents,
because iomap calls iov_iter_zero() and this seems to ignore the value
of ->nofault.
Is that intentional? I can get around it by surrounding the iomap call
with pagefault_disable() / pagefault_enable(), but it seems odd to do
so, given that iov_iter->nofault was set to true.

[1] https://git.kernel.org/pub/scm/fs/xfs/xfstests-dev.git/commit/?id=d3cbdabffc4cb28850e97bc7bd8a7a1460db94e5

Thanks.

The first two patches are independent of the core of this patch queue
and I've asked the respective maintainers to have a look, but I've not
heard back from them.  The first patch should just go into Al's tree;
it's a relatively straight-forward fix.  The second patch really needs
to be looked at; it might break things:

  iov_iter: Fix iov_iter_get_pages{,_alloc} page fault return value
  powerpc/kvm: Fix kvm_use_magic_page


Al and Linus seem to have a disagreement about the error reporting
semantics that functions fault_in_{readable,writeable} and
fault_in_iov_iter_{readable,writeable} should have.  I've implemented
Linus's suggestion of returning the number of bytes not faulted in and I
think that being able to tell if "nothing", "something" or "everything"
could be faulted in does help, but I'll live with anything that allows
us to make progress.


The iomap changes should ideally be reviewed by Christoph; I've not
heard from him about those.


Thanks,
Andreas

Andreas Gruenbacher (16):
  iov_iter: Fix iov_iter_get_pages{,_alloc} page fault return value
  powerpc/kvm: Fix kvm_use_magic_page
  gup: Turn fault_in_pages_{readable,writeable} into
    fault_in_{readable,writeable}
  iov_iter: Turn iov_iter_fault_in_readable into
    fault_in_iov_iter_readable
  iov_iter: Introduce fault_in_iov_iter_writeable
  gfs2: Add wrapper for iomap_file_buffered_write
  gfs2: Clean up function may_grant
  gfs2: Move the inode glock locking to gfs2_file_buffered_write
  gfs2: Eliminate ip->i_gh
  gfs2: Fix mmap + page fault deadlocks for buffered I/O
  iomap: Fix iomap_dio_rw return value for user copies
  iomap: Support partial direct I/O on user copy failures
  iomap: Add done_before argument to iomap_dio_rw
  gup: Introduce FOLL_NOFAULT flag to disable page faults
  iov_iter: Introduce nofault flag to disable page faults
  gfs2: Fix mmap + page fault deadlocks for direct I/O

Bob Peterson (3):
  gfs2: Eliminate vestigial HIF_FIRST
  gfs2: Remove redundant check from gfs2_glock_dq
  gfs2: Introduce flag for glock holder auto-demotion

 arch/powerpc/kernel/kvm.c           |   3 +-
 arch/powerpc/kernel/signal_32.c     |   4 +-
 arch/powerpc/kernel/signal_64.c     |   2 +-
 arch/x86/kernel/fpu/signal.c        |   7 +-
 drivers/gpu/drm/armada/armada_gem.c |   7 +-
 fs/btrfs/file.c                     |   7 +-
 fs/btrfs/ioctl.c                    |   5 +-
 fs/ext4/file.c                      |   5 +-
 fs/f2fs/file.c                      |   2 +-
 fs/fuse/file.c                      |   2 +-
 fs/gfs2/bmap.c                      |  60 +----
 fs/gfs2/file.c                      | 245 ++++++++++++++++++--
 fs/gfs2/glock.c                     | 340 +++++++++++++++++++++-------
 fs/gfs2/glock.h                     |  20 ++
 fs/gfs2/incore.h                    |   5 +-
 fs/iomap/buffered-io.c              |   2 +-
 fs/iomap/direct-io.c                |  21 +-
 fs/ntfs/file.c                      |   2 +-
 fs/xfs/xfs_file.c                   |   6 +-
 fs/zonefs/super.c                   |   4 +-
 include/linux/iomap.h               |  11 +-
 include/linux/mm.h                  |   3 +-
 include/linux/pagemap.h             |  58 +----
 include/linux/uio.h                 |   4 +-
 lib/iov_iter.c                      | 103 +++++++--
 mm/filemap.c                        |   4 +-
 mm/gup.c                            | 139 +++++++++++-
 27 files changed, 785 insertions(+), 286 deletions(-)

--
2.26.3

-- 
Filipe David Manana,

“Whether you think you can, or you think you can't — you're right.”