Re: [PATCH v2 14/20] mm: Provide speculative fault infrastructure

[Peter Zijlstra]

On Sun, Aug 27, 2017 at 03:18:23AM +0300, Kirill A. Shutemov wrote:
> On Fri, Aug 18, 2017 at 12:05:13AM +0200, Laurent Dufour wrote:
> > +	/*
> > +	 * Can't call vm_ops service has we don't know what they would do
> > +	 * with the VMA.
> > +	 * This include huge page from hugetlbfs.
> > +	 */
> > +	if (vma->vm_ops)
> > +		goto unlock;
> 
> I think we need to have a way to white-list safe ->vm_ops.

Either that, or simply teach all ->fault() callbacks about speculative
faults. Shouldn't be too hard, just 'work'.

> > +
> > +	if (unlikely(!vma->anon_vma))
> > +		goto unlock;
> 
> It deserves a comment.

Yes, that was very much not intended. It wrecks most of the fun. This
really _should_ work for file maps too.

> > +	/*
> > +	 * Do a speculative lookup of the PTE entry.
> > +	 */
> > +	local_irq_disable();
> > +	pgd = pgd_offset(mm, address);
> > +	if (pgd_none(*pgd) || unlikely(pgd_bad(*pgd)))
> > +		goto out_walk;
> > +
> > +	p4d = p4d_alloc(mm, pgd, address);
> > +	if (p4d_none(*p4d) || unlikely(p4d_bad(*p4d)))
> > +		goto out_walk;
> > +
> > +	pud = pud_alloc(mm, p4d, address);
> > +	if (pud_none(*pud) || unlikely(pud_bad(*pud)))
> > +		goto out_walk;
> > +
> > +	pmd = pmd_offset(pud, address);
> > +	if (pmd_none(*pmd) || unlikely(pmd_bad(*pmd)))
> > +		goto out_walk;
> > +
> > +	/*
> > +	 * The above does not allocate/instantiate page-tables because doing so
> > +	 * would lead to the possibility of instantiating page-tables after
> > +	 * free_pgtables() -- and consequently leaking them.
> > +	 *
> > +	 * The result is that we take at least one !speculative fault per PMD
> > +	 * in order to instantiate it.
> > +	 */
> 
> 
> Doing all this job and just give up because we cannot allocate page tables
> looks very wasteful to me.
> 
> Have you considered to look how we can hand over from speculative to
> non-speculative path without starting from scratch (when possible)?

So we _can_ in fact allocate and install page-tables, but we have to be
very careful about it. The interesting case is where we race with
free_pgtables() and install a page that was just taken out.

But since we already have the VMA I think we can do something like:

	if (p*g_none()) {
		p*d_t *new = p*d_alloc_one(mm, address);

		spin_lock(&mm->page_table_lock);
		if (!vma_changed_or_dead(vma,seq)) {
			if (p*d_none())
				p*d_populate(mm, p*d, new);
			else
				p*d_free(new);

			new = NULL;
		}
		spin_unlock(&mm->page_table_lock);

		if (new) {
			p*d_free(new);
			goto out_walk;
		}
	}

I just never bothered with that, figured we ought to get the basics
working before trying to be clever.

> > +	/* Transparent huge pages are not supported. */
> > +	if (unlikely(pmd_trans_huge(*pmd)))
> > +		goto out_walk;
> 
> That's looks like a blocker to me.
> 
> Is there any problem with making it supported (besides plain coding)?

Not that I can remember, but I never really looked at THP, I don't think
we even had that when I did the first versions.