Re: [PATCH v3] mm: Avoid unnecessary page fault retires on shared memory types

Peter Xu
2022-05-27T14:53:56Z

On Fri, May 27, 2022 at 12:46:31PM +0200, Ingo Molnar wrote:
>
> * Peter Xu <peterx@redhat.com> wrote:
>
> > This patch provides a ~12% perf boost on my aarch64 test VM with a simple
> > program sequentially dirtying 400MB shmem file being mmap()ed and these are
> > the time it needs:
> >
> > Before: 650.980 ms (+-1.94%)
> > After: 569.396 ms (+-1.38%)
>
> Nice!
>
> > arch/x86/mm/fault.c | 4 ++++
>
> Reviewed-by: Ingo Molnar <mingo@kernel.org>
>
> Minor comment typo:
>
> > + /*
> > + * We should do the same as VM_FAULT_RETRY, but let's not
> > + * return -EBUSY since that's not reflecting the reality on
> > + * what has happened - we've just fully completed a page
> > + * fault, with the mmap lock released. Use -EAGAIN to show
> > + * that we want to take the mmap lock _again_.
> > + */
>
> s/reflecting the reality on what has happened
> /reflecting the reality of what has happened

Will fix.

>
> > ret = handle_mm_fault(vma, address, fault_flags, NULL);
> > +
> > + if (ret & VM_FAULT_COMPLETED) {
> > + /*
> > + * NOTE: it's a pity that we need to retake the lock here
> > + * to pair with the unlock() in the callers. Ideally we
> > + * could tell the callers so they do not need to unlock.
> > + */
> > + mmap_read_lock(mm);
> > + *unlocked = true;
> > + return 0;
>
> Indeed that's a pity - I guess more performance could be gained here,
> especially in highly parallel threaded workloads?

Yes I think so.

The patch avoids the page fault retry, including the mmap lock/unlock side.
Now if we retake the lock for fixup_user_fault() we still safe time for
pgtable walks but the lock overhead will be somehow kept, just with smaller
critical sections.

Some fixup_user_fault() callers won't be affected as long as unlocked==NULL
is passed - e.g. the futex code path (fault_in_user_writeable). After all
they never needed to retake the lock before/after this patch.

It's about the other callers, and they may need some more touch-ups case by
case. Examples are follow_fault_pfn() in vfio and hva_to_pfn_remapped() in
KVM: both of them returns -EAGAIN when *unlocked==true. We need to teach
them to know "*unlocked==true" does not necessarily mean a retry attempt.

I think I can look into them if this patch can be accepted as a follow up.

Thanks for taking a look!

--
Peter Xu