Messages in this thread | | | Date | Mon, 10 Aug 2020 18:53:59 +0300 | From | Mike Rapoport <> | Subject | Re: [PATCH] x86/mm/64: Do not dereference non-present PGD entries |
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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.
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