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

[Suren Baghdasaryan]

On Fri, Apr 26, 2024 at 7:00 AM Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote:
>
> 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@xxxxxxxxx/
>
> I can't see why it's that good.  It shouldn't be that good.  I'm
> seeing big numbers here:
>
>       4366 ą  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?

That indeed looks overly good. Sorry, I didn't have a chance to run
the benchmarks on my side yet because of the ongoing Android bootcamp
this week.

>
> 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 ą  2%     +94.0       97.12        perf-profile.calltracecycles-pp.osq_lock.rwsem_optimistic_spin.rwsem_down_write_slowpath.down_write_killable.vm_mmap_pgoff
>       3.45 ą  2%     +94.1       97.59        perf-profile.calltracecycles-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 ą  2%  perf-stat.ps.context-switches
>     663.67 ą  5%   +7551.9%      50783        vm-scalability.time.involuntary_context_switches
>  1.105e+08           -86.7%   14697764 ą  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?

I think the only path in either do_anonymous_page() or
do_huge_pmd_anonymous_page() that skips calling anon_vma_prepare() is
the "Use the zero-page for reads" here:
https://elixir.bootlin.com/linux/latest/source/mm/memory.c#L4265. I
didn't look into this particular benchmark yet but will try it out
once I have some time to benchmark your change.

>