Re: [PATCH] mm: Always sanity check anon_vma first for per-vma locks

[Matthew Wilcox <willy@infradead.org>]

On Fri, Apr 12, 2024 at 04:14:16AM +0100, Matthew Wilcox wrote:
> Suren, what would you think to this?
> 
> diff --git a/mm/memory.c b/mm/memory.c
> index 6e2fe960473d..e495adcbe968 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -5821,15 +5821,6 @@ struct vm_area_struct *lock_vma_under_rcu(struct mm_struct *mm,
>         if (!vma_start_read(vma))
>                 goto inval;
> 
> -       /*
> -        * find_mergeable_anon_vma uses adjacent vmas which are not locked.
> -        * This check must happen after vma_start_read(); otherwise, a
> -        * concurrent mremap() with MREMAP_DONTUNMAP could dissociate the VMA
> -        * from its anon_vma.
> -        */
> -       if (unlikely(vma_is_anonymous(vma) && !vma->anon_vma))
> -               goto inval_end_read;
> -
>         /* Check since vm_start/vm_end might change before we lock the VMA */
>         if (unlikely(address < vma->vm_start || address >= vma->vm_end))
>                 goto inval_end_read;
> 
> That takes a few insns out of the page fault path (good!) at the cost
> of one extra trip around the fault handler for the first fault on an
> anon vma.  It makes the file & anon paths more similar to each other
> (good!)
> 
> We'd need some data to be sure it's really a win, but less code is
> always good.

Intel's 0day got back to me with data and it's ridiculously good.
Headline figure: over 3x throughput improvement with vm-scalability
https://lore.kernel.org/all/202404261055.c5e24608-oliver.sang@intel.com/

I can't see why it's that good.  It shouldn't be that good.  I'm
seeing big numbers here:

      4366 ±  2%    +565.6%      29061        perf-stat.overall.cycles-between-cache-misses

and the code being deleted is only checking vma->vm_ops and
vma->anon_vma.  Surely that cache line is referenced so frequently
during pagefault that deleting a reference here will make no difference
at all?

We've clearly got an inlining change.  viz:

     72.57           -72.6        0.00        perf-profile.calltrace.cycles-pp.exc_page_fault.asm_exc_page_fault.do_access
     73.28           -72.6        0.70        perf-profile.calltrace.cycles-pp.asm_exc_page_fault.do_access
     72.55           -72.5        0.00        perf-profile.calltrace.cycles-pp.do_user_addr_fault.exc_page_fault.asm_exc_page_fault.do_access
     69.93           -69.9        0.00        perf-profile.calltrace.cycles-pp.lock_mm_and_find_vma.do_user_addr_fault.exc_page_fault.asm_exc_page_fault.do_access
     69.12           -69.1        0.00        perf-profile.calltrace.cycles-pp.down_read_killable.lock_mm_and_find_vma.do_user_addr_fault.exc_page_fault.asm_exc_page_fault
     68.78           -68.8        0.00        perf-profile.calltrace.cycles-pp.rwsem_down_read_slowpath.down_read_killable.lock_mm_and_find_vma.do_user_addr_fault.exc_page_fault
     65.78           -65.8        0.00        perf-profile.calltrace.cycles-pp._raw_spin_lock_irq.rwsem_down_read_slowpath.down_read_killable.lock_mm_and_find_vma.do_user_addr_fault
     65.43           -65.4        0.00        perf-profile.calltrace.cycles-pp.native_queued_spin_lock_slowpath._raw_spin_lock_irq.rwsem_down_read_slowpath.down_read_killable.lock_mm_and_find_vma

     11.22           +86.5       97.68        perf-profile.calltrace.cycles-pp.down_write_killable.vm_mmap_pgoff.ksys_mmap_pgoff.do_syscall_64.entry_SYSCALL_64_after_hwframe
     11.14           +86.5       97.66        perf-profile.calltrace.cycles-pp.rwsem_down_write_slowpath.down_write_killable.vm_mmap_pgoff.ksys_mmap_pgoff.do_syscall_64
      3.17 ±  2%     +94.0       97.12        perf-profile.calltrace.cycles-pp.osq_lock.rwsem_optimistic_spin.rwsem_down_write_slowpath.down_write_killable.vm_mmap_pgoff
      3.45 ±  2%     +94.1       97.59        perf-profile.calltrace.cycles-pp.rwsem_optimistic_spin.rwsem_down_write_slowpath.down_write_killable.vm_mmap_pgoff.ksys_mmap_pgoff
      0.00           +98.2       98.15        perf-profile.calltrace.cycles-pp.vm_mmap_pgoff.ksys_mmap_pgoff.do_syscall_64.entry_SYSCALL_64_after_hwframe
      0.00           +98.2       98.16        perf-profile.calltrace.cycles-pp.ksys_mmap_pgoff.do_syscall_64.entry_SYSCALL_64_after_hwframe

so maybe the compiler has been able to eliminate some loads from
contended cachelines?

    703147           -87.6%      87147 ±  2%  perf-stat.ps.context-switches
    663.67 ±  5%   +7551.9%      50783        vm-scalability.time.involuntary_context_switches
 1.105e+08           -86.7%   14697764 ±  2%  vm-scalability.time.voluntary_context_switches

indicates to me that we're taking the mmap rwsem far less often (those
would be accounted as voluntary context switches).

So maybe the cache miss reduction is a consequence of just running for
longer before being preempted.

I still don't understand why we have to take the mmap_sem less often.
Is there perhaps a VMA for which we have a NULL vm_ops, but don't set
an anon_vma on a page fault?