Re: [RFC PATCH 24/37] mm: implement speculative handling in __do_fault()

[Peter Zijlstra]

On Thu, Apr 08, 2021 at 08:13:43AM +0100, Matthew Wilcox wrote:
> On Thu, Apr 08, 2021 at 09:00:26AM +0200, Peter Zijlstra wrote:
> > On Wed, Apr 07, 2021 at 10:27:12PM +0100, Matthew Wilcox wrote:
> > > Doing I/O without any lock held already works; it just uses the file
> > > refcount.  It would be better to use a vma refcount, as I already said.
> > 
> > The original workload that I developed SPF for (waaaay back when) was
> > prefaulting a single huge vma. Using a vma refcount was a total loss
> > because it resulted in the same cacheline contention that down_read()
> > was having.
> > 
> > As such, I'm always incredibly sad to see mention of vma refcounts.
> > They're fundamentally not solving the problem :/
> 
> OK, let me outline my locking scheme because I think it's rather better
> than Michel's.  The vma refcount is the slow path.
> 
> 1. take the RCU read lock
> 2. walk the pgd/p4d/pud/pmd
> 3. allocate page tables if necessary.  *handwave GFP flags*.

The problem with allocating page-tables was that you can race with
zap_page_range() if you're not holding mmap_sem, and as such can install
a page-table after, in which case it leaks.

IIRC that was solvable, but it did need a bit of care.

> 4. walk the vma tree
> 5. call ->map_pages

I can't remember ->map_pages().. I think that's 'new'. git-blame tells
me that's 2014, and I did the original SPF in 2010.

Yes, that looks like a useful thing to have, it does the non-blocking
part of ->fault().

I suppose the thing missing here is that if ->map_pages() does not
return a page, we have:

  goto 9

> 6. take ptlock
> 7. insert page(s)
> 8. drop ptlock
> if this all worked out, we're done, drop the RCU read lock and return.

> 9. increment vma refcount
> 10. drop RCU read lock
> 11. call ->fault
> 12. decrement vma refcount

And here we do 6-8 again, right?

> Compared to today, where we bump the refcount on the file underlying the
> vma, this is _better_ scalability -- different mappings of the same file
> will not contend on the file's refcount.
>
> I suspect your huge VMA was anon, and that wouldn't need a vma refcount
> as faulting in new pages doesn't need to do I/O, just drop the RCU
> lock, allocate and retry.

IIRC yes, it was either a huge matrix setup or some database thing, I
can't remember. But the thing was, we didn't have that ->map_pages(), so
we had to call ->fault(), which can sleep, so I had to use SRCU across
the whole thing (or rather, I hacked up preemptible-rcu, because SRCU
was super primitive back then). It did kick start significant SRCU
rework IIRC. Anyway, that's all ancient history.