x86/mm/64: Update comment in preallocate_vmalloc_pages()

The comment explaining why 4-level systems only need to allocate on
the P4D level caused some confustion. Update it to better explain why
on 4-level systems the allocation on PUD level is necessary.

Signed-off-by: Joerg Roedel <jroedel@suse.de>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20200814151947.26229-3-joro@8bytes.org

diff --git a/arch/x86/mm/init_64.c b/arch/x86/mm/init_64.c
index 777d835467648b9b820101b3972d6bd1f20d0197..b5a3fa4033d38bf00fd4766683459892c9554ec8 100644 (file)
--- a/arch/x86/mm/init_64.c
+++ b/arch/x86/mm/init_64.c
@@ -1252,14 +1252,19 @@ static void __init preallocate_vmalloc_pages(void)
                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;
 
+               /*
+                * 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 on 4-level systems too.
+                */
                lvl = "pud";
                pud = pud_alloc(&init_mm, p4d, addr);
                if (!pud)