On Sun, Dec 16, 2012 at 1:00 AM, Ingo Molnar [off-list ref] wrote:

* Andy Lutomirski [off-list ref] wrote:

quoted

This is a serious cause of mmap_sem contention.  MAP_POPULATE
and MCL_FUTURE, in particular, are disastrous in multithreaded programs.

Signed-off-by: Andy Lutomirski <luto@amacapital.net>
---

Changes from v1:

The non-unlocking versions of do_mmap_pgoff and mmap_region are still
available for aio_setup_ring's benefit.  In theory, aio_setup_ring
would do better with a lock-downgrading version, but that would be
somewhat ugly and doesn't help my workload.

 arch/tile/mm/elf.c |  9 +++---
 fs/aio.c           |  4 +++
 include/linux/mm.h | 19 ++++++++++--
 ipc/shm.c          |  6 ++--
 mm/fremap.c        | 10 ++++--
 mm/mmap.c          | 89 ++++++++++++++++++++++++++++++++++++++++++++++++------
 mm/util.c          |  3 +-
 7 files changed, 117 insertions(+), 23 deletions(-)

quoted

+unsigned long mmap_region(struct file *file, unsigned long addr,
+                       unsigned long len, unsigned long flags,
+                       vm_flags_t vm_flags, unsigned long pgoff)
+{
+     return mmap_region_helper(file, addr, len, flags, vm_flags, pgoff, 0);
+}
+

That 0 really wants to be NULL ...

Sigh.  I blame C++11 -- I wanted to type nullptr, but that's no good :)

Also, with your patch applied there's no user of mmap_region()
left anymore.

More fundamentally, while I agree with the optimization,
couldn't we de-uglify it a bit more?

In particular, instead of this wrappery:

quoted

+unsigned long mmap_region_unlock(struct file *file, unsigned long addr,
+                              unsigned long len, unsigned long flags,
+                              vm_flags_t vm_flags, unsigned long pgoff)
+{
+     int downgraded = 0;
+     unsigned long ret = mmap_region_helper(file, addr, len,
+             flags, vm_flags, pgoff, &downgraded);
+
+     if (downgraded)
+             up_read(&current->mm->mmap_sem);
+     else
+             up_write(&current->mm->mmap_sem);
+
+     return ret;
+}

1)

We could at minimum wrap up the conditional unlocking as:

        up_read_write(&mm->mmap_sem, read_locked);

With that I'd also suggest to rename 'downgraded' to
'read_locked', which more clearly expresses the locking state.

2)

More aggressively, we could just make it the _rule_ that the mm
lock gets downgraded to read in mmap_region_helper(), no matter
what.

From a quick look I *think* all the usage sites (including
sys_aio_setup()) are fine with that unlocking - but I could be
wrong.

They are.

There's a couple of shorter codepaths that would now see an
extra op of downgrading:

        down_write(&mm->mmap_sem);
        ...
        downgrade_write(&mm->mmap_sem);
        ...
        up_read(&mm->mmap_sem);

with not much work done with the lock read-locked - but I think
they are all fine and mostly affect error paths. So there's no
real value in keeping the conditional nature of the unlocking I
think.

There's also the normal (i.e. neither lock nor populate) success path.
 Does this matter?  Presumably downgrade_write + up_read isn't much
slower than up_write.

--Andy

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