On Sun, Dec 16, 2012 at 1:00 AM, Ingo Molnar <mingo@xxxxxxxxxx> wrote:
>
> * Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
>
>> 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@xxxxxxxxxxxxxx>
>> ---
>>
>> 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(-)
>
>> +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:
>
>> +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(¤t->mm->mmap_sem);
>> + else
>> + up_write(¤t->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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