linux-kernel - [PATCH 2/2] x86/mm/64: Update comment in preallocate_vmalloc

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Date:   Fri, 14 Aug 2020 17:19:47 +0200
From:   Joerg Roedel <joro@...tes.org>
To:     x86@...nel.org
Cc:     Ingo Molnar <mingo@...nel.org>, Mike Rapoport <rppt@...ux.ibm.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>, hpa@...or.com,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Jason@...c4.com, Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org, kirill.shutemov@...el.com,
        Joerg Roedel <jroedel@...e.de>
Subject: [PATCH 2/2] x86/mm/64: Update comment in preallocate_vmalloc_pages()

From: Joerg Roedel <jroedel@...e.de>

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@...e.de>
---
 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 777d83546764..124e63795ac9 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)
-- 
2.28.0