Messages in this thread | | | Subject | Re: [PATCH v8 0/9] rwsem performance optimizations | From | Davidlohr Bueso <> | Date | Wed, 09 Oct 2013 22:03:57 -0700 |
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On Wed, 2013-10-09 at 20:14 -0700, Linus Torvalds wrote: > On Wed, Oct 9, 2013 at 12:28 AM, Peter Zijlstra <peterz@infradead.org> 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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