[PATCH 0/2] Reduce mmap_sem hold times during file backed page faults

[Michel Lespinasse <walken@google.com>]

Linus, I would appreciate your comments on this since you shot down the
previous proposal. I hope you'll find this approach is sane, but I would
be interested to hear if you have specific objections.

mmap_sem is very coarse grained (per process) and has long read-hold times
(disk latencies); this breaks down rapidly for workloads that use both
read and write mmap_sem acquires. This short patch series tries to reduce
mmap_sem hold times when faulting in file backed VMAs.

First patch creates a single place to lock the page in filemap_fault().
There should be no behavior differences.

Second patch modifies that lock_page() so that, if trylock_page() fails,
we consider releasing the mmap_sem while waiting for page to be unlocked.
This is controlled by a new FAULT_FLAG_RELEASE flag. If the mmap_sem gets
released, we return the VM_FAULT_RELEASED status; the caller is then expected
to re-acquire mmap_sem and retry the page fault. Chances are that the same
page will be accessed and will now be unlocked, so the mmap_sem hold time
will be short.

Michel Lespinasse (2):
  Unique path for locking page in filemap_fault()
  Release mmap_sem when page fault blocks on disk transfer.

 arch/x86/mm/fault.c |   35 ++++++++++++++++++++++++++---------
 include/linux/mm.h  |    2 ++
 mm/filemap.c        |   38 +++++++++++++++++++++++++++++---------
 mm/memory.c         |    3 ++-
 4 files changed, 59 insertions(+), 19 deletions(-)

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