[RFC PATCH v1 0/2] mm: use VMA lock for kernel faults on user addresses

Right now, kernel faults on user addresses, such as those from
copy_from_user() and copy_to_user(), unconditionally fall back
to the mmap_lock path.

This patchset switches them to the per-VMA lock path for three
reasons:

1. These faults are common. On a typical Ubuntu system, hundreds
   of kernel faults on user addresses occur every second in
   python3, apt-esm-hook, package-data-do, teamviewerd, bash,
   scudo, cscope, gnome-shell, systemd, and many other
   applications. Handling these faults under mmap_lock
   unnecessarily increases lock contention.

2. It removes one obstacle to simplifying filemap_fault().
   Matthew has proposed removing the page fault retry path and
   performing I/O while holding locks [1]. Based on that
   approach, Hongru has already reported regressions caused by
   performing I/O under mmap_lock for kernel faults on user
   addresses [2]. This patchset removes that obstacle.

   Another source of I/O under mmap_lock is GUP, which also
   relies on mmap_lock today and has been reported by Hongru to
   exhibit similar regressions [2]. It appears Rik van Riel may
   be addressing this separately [3].

3. The current implementation is inconsistent. Although kernel
   faults on user addresses always fall back to mmap_lock,
   arch/*/mm/fault.c still performs !user_mode checks in the
   per-VMA lock path, making that code effectively dead.

As an RFC, this patchset demonstrates the approach on x86 and
arm64 only. Other architectures will need to be updated as well.

[1] https://lore.kernel.org/linux-mm/[email protected]/
[2] https://lore.kernel.org/linux-mm/[email protected]/
[3] https://lore.kernel.org/linux-mm/[email protected]/

Barry Song (Xiaomi) (2):
  x86/mm: use VMA lock for kernel faults on user addresses
  arm64/mm: use VMA lock for kernel faults on user addresses

 arch/arm64/mm/fault.c | 4 +++-
 arch/x86/mm/fault.c   | 3 ---
 2 files changed, 3 insertions(+), 4 deletions(-)

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