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.
