On Mon, Aug 10, 2020 at 07:27:33AM -0700, Dave Hansen wrote:
> ... adding Kirill
>
> On 8/7/20 1:40 AM, Joerg Roedel wrote:
> > + lvl = "p4d";
> > + p4d = p4d_alloc(&init_mm, pgd, addr);
> > + if (!p4d)
> > + goto failed;
> >
> > + /*
> > + * With 5-level paging the P4D level is not folded. So the PGDs
> > + * are now populated and there is no need to walk down to the
> > + * PUD level.
> > + */
> > if (pgtable_l5_enabled())
> > continue;
>
> It's early and I'm a coffee or two short of awake, but I had to stare at
> the comment for a but to make sense of it.
>
> It feels wrong, I think, because the 5-level code usually ends up doing
> *more* allocations and in this case, it is _appearing_ to do fewer.
> Would something like this make sense?
Unless I miss something, with 5 levels vmalloc mappings are shared at
p4d level, so allocating a p4d page would be enough. With 4 levels,
p4d_alloc() is a nop and pud is the first actually populated level below
pgd.
> /*
> * The goal here is to allocate all possibly required
> * hardware page tables pointed to by the top hardware
> * level.
> *
> * On 4-level systems, the p4d layer is folded away and
> * the above code does no preallocation. Below, go down
> * to the pud _software_ level to ensure the second
> * hardware level is allocated.
> */
>
>
> > - pud = pud_offset(p4d, addr);
> > - if (pud_none(*pud)) {
> > - /* Ends up here only with 4-level paging */
> > - pud = pud_alloc(&init_mm, p4d, addr);
> > - if (!pud) {
> > - lvl = "pud";
> > - goto failed;
> > - }
> > - }
> > + lvl = "pud";
> > + pud = pud_alloc(&init_mm, p4d, addr);
> > + if (!pud)
> > + goto failed;
> > }
--
Sincerely yours,
Mike.
