On Wed, 2013-10-09 at 20:14 -0700, Linus Torvalds wrote:
> On Wed, Oct 9, 2013 at 12:28 AM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> >
> > The workload that I got the report from was a virus scanner, it would
> > spawn nr_cpus threads and {mmap file, scan content, munmap} through your
> > filesystem.
>
> So I suspect we could make the mmap_sem write area *much* smaller for
> the normal cases.
>
> Look at do_mmap_pgoff(), for example: it is run entirely under
> mmap_sem, but 99% of what it does doesn't actually need the lock.
>
> The part that really needs the lock is
>
> addr = get_unmapped_area(file, addr, len, pgoff, flags);
> addr = mmap_region(file, addr, len, vm_flags, pgoff);
>
> but we hold it over all the other stuff too.
>
True. By looking at the callers, we're always doing:
down_write(&mm->mmap_sem);
do_mmap_pgoff()
...
up_write(&mm->mmap_sem);
That goes for shm, aio, and of course mmap_pgoff().
While I know you hate two level locking, one way to go about this is to
take the lock inside do_mmap_pgoff() after the initial checks (flags,
page align, etc.) and return with the lock held, leaving the caller to
unlock it.
> In fact, even if we moved the mmap_sem down into do_mmap(), and moved
> code around a bit to only hold it over those functions, it would still
> cover unnecessarily much. For example, while merging is common, not
> merging is pretty common too, and we do that
>
> vma = kmem_cache_zalloc(vm_area_cachep, GFP_KERNEL);
>
> allocation under the lock. We could easily do things like preallocate
> it outside the lock.
>
AFAICT there are also checks that should be done at the beginning of the
function, such as checking for MAP_LOCKED and VM_LOCKED flags before
calling get_unmapped_area().
Thanks,
Davidlohr
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