[PATCH 0/2] Add a 'seqcount' between gup_fast and copy_page_range

[Jason Gunthorpe <jgg@nvidia.com>]

As discussed and suggested by Linus use a seqcount like thing to close the
small race between gup_fast and copy_page_range.

Unfortunately the good suggestion to just use write_seqcount_begin() blows
up lockdep immediately due to the (new?) requirement that the write side
of seqcount be in a preempt disabled region. For this application it does
not seem like a good idea, nor is it necessary as we don't spin on retry.

So I open coded a similar construct. Don't like it, will redo this in some
other way if there is a better idea. Since seqcount seems to have this
property now, it also feels wrong to be the only place to use the raw_
functions specifically to avoid the lockdep checks and other parts of
seqcount on the read side.

This can go after the merge window. I was table to test it using two
threads, one forking and the other using ibv_reg_mr() to trigger GUP
fast. Modifying copy_page_range() to sleep made the window large enough to
reliably hit to test the logic.

Jason Gunthorpe (2):
  mm: reorganize internal_get_user_pages_fast()
  mm: prevent gup_fast from racing with COW during fork

 include/linux/mm_types.h |   6 +++
 kernel/fork.c            |   1 +
 mm/gup.c                 | 107 ++++++++++++++++++++++++---------------
 mm/memory.c              |  16 +++++-
 4 files changed, 87 insertions(+), 43 deletions(-)

-- 
2.28.0