vma_start_write_killable

When we added the VMA lock, we made a major oversight in not adding a
killable variant.  That can run us into trouble where a thread takes
the VMA lock for read (eg handling a page fault) and then goes out to
lunch for an hour (eg doing reclaim).  Another thread tries to modify
the VMA, taking the mmap_lock for write, then attempts to lock the VMA
for write.  That blocks on the first thread, and ensures that every
other page fault now tries to take the mmap_lock for read.  Because
everything's in an uninterruptible sleep, we can't kill the task,
which makes me angry.

This patch set just adds vma_start_write_killable() and converts one
caller to use it.  Most users are somewhat tricky to convert, so expect
follow-up individual patches per call-site which need careful analysis
to make sure we've done proper cleanup.

Matthew Wilcox (Oracle) (2):
  mm: Add vma_start_write_killable()
  mm: Use vma_start_write_killable() in dup_mmap()

 include/linux/mmap_lock.h        | 31 +++++++++++++++++++++++++++++--
 mm/mmap.c                        | 12 +++---------
 mm/mmap_lock.c                   | 27 ++++++++++++++++++---------
 tools/testing/vma/vma_internal.h |  8 ++++++++
 4 files changed, 58 insertions(+), 20 deletions(-)

-- 
2.47.2