linux-kernel - [PATCH] x86/mm: Simplify conditionals in PGD synchronization functions

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Date: Tue,  4 Jun 2024 09:44:13 +0800
From: AceLan Kao <acelan.kao@...onical.com>
To: Dave Hansen <dave.hansen@...ux.intel.com>,
	Andy Lutomirski <luto@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	Borislav Petkov <bp@...en8.de>,
	x86@...nel.org,
	"H. Peter Anvin" <hpa@...or.com>,
	linux-kernel@...r.kernel.org
Subject: [PATCH] x86/mm: Simplify conditionals in PGD synchronization functions

From: "Chia-Lin Kao (AceLan)" <acelan.kao@...onical.com>

This patch streamlines the conditional checks within the sync_global_pgds_l5
and sync_global_pgds_l4 functions. Previously, the code redundantly checked
both reference and target PGD/P4D entries for existence before proceeding
with further actions. This redundancy is unnecessary since the existence of
the reference entries is implied by earlier checks in the code flow.

By assuming the reference PGD/P4D entries (*pgd_ref and *p4d_ref) are always
valid within these conditional checks, we simplify the logic to only check
the target entries (*pgd and *p4d). This change makes the code easier to read
and understand while maintaining its original functionality.

Modifications:
- In sync_global_pgds_l5, removed the redundant check for pgd_none(*pgd_ref)
  when asserting or setting *pgd entries.
- Applied a similar simplification in sync_global_pgds_l4 for *p4d and *p4d_ref
  checks.

This patch does not alter the functionality of these functions but improves
the clarity and efficiency of the code by removing unnecessary conditions.

Signed-off-by: Chia-Lin Kao (AceLan) <acelan.kao@...onical.com>
---
 arch/x86/mm/init_64.c | 10 ++++------
 1 file changed, 4 insertions(+), 6 deletions(-)

diff --git a/arch/x86/mm/init_64.c b/arch/x86/mm/init_64.c
index a0dffaca6d2b..ed964bd081f8 100644
--- a/arch/x86/mm/init_64.c
+++ b/arch/x86/mm/init_64.c
@@ -155,10 +155,9 @@ static void sync_global_pgds_l5(unsigned long start, unsigned long end)
 			pgt_lock = &pgd_page_get_mm(page)->page_table_lock;
 			spin_lock(pgt_lock);
 
-			if (!pgd_none(*pgd_ref) && !pgd_none(*pgd))
+			if (!pgd_none(*pgd))
 				BUG_ON(pgd_page_vaddr(*pgd) != pgd_page_vaddr(*pgd_ref));
-
-			if (pgd_none(*pgd))
+			else
 				set_pgd(pgd, *pgd_ref);
 
 			spin_unlock(pgt_lock);
@@ -198,11 +197,10 @@ static void sync_global_pgds_l4(unsigned long start, unsigned long end)
 			pgt_lock = &pgd_page_get_mm(page)->page_table_lock;
 			spin_lock(pgt_lock);
 
-			if (!p4d_none(*p4d_ref) && !p4d_none(*p4d))
+			if (!p4d_none(*p4d))
 				BUG_ON(p4d_pgtable(*p4d)
 				       != p4d_pgtable(*p4d_ref));
-
-			if (p4d_none(*p4d))
+			else
 				set_p4d(p4d, *p4d_ref);
 
 			spin_unlock(pgt_lock);
-- 
2.34.1