[tip: x86/mm] x86/mm/64: Update comment in preallocate_vmalloc_pages()

[tip-bot2 for Joerg Roedel]

The following commit has been merged into the x86/mm branch of tip:

Commit-ID:     7a27ef5e83089090f3a4073a9157c862ef00acfc
Gitweb:        https://git.kernel.org/tip/7a27ef5e83089090f3a4073a9157c862ef00acfc
Author:        Joerg Roedel <jroedel@xxxxxxx>
AuthorDate:    Fri, 14 Aug 2020 17:19:47 +02:00
Committer:     Ingo Molnar <mingo@xxxxxxxxxx>
CommitterDate: Sat, 15 Aug 2020 13:56:16 +02:00

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@xxxxxxx>
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
Link: https://lore.kernel.org/r/20200814151947.26229-3-joro@xxxxxxxxxx
---
 arch/x86/mm/init_64.c | 15 ++++++++++-----
 1 file changed, 10 insertions(+), 5 deletions(-)

diff --git a/arch/x86/mm/init_64.c b/arch/x86/mm/init_64.c
index 777d835..b5a3fa4 100644
--- 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)