linux-kernel - [PATCH v7 07/15] x86/mm: Physical address comparisons in fill

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Message-ID: <a24dbd06a8ab0c556711971a0f15503dc575eee9.1765386422.git.m.wieczorretman@pm.me>
Date: Wed, 10 Dec 2025 17:29:29 +0000
From: Maciej Wieczor-Retman <m.wieczorretman@...me>
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>
Cc: m.wieczorretman@...me, Maciej Wieczor-Retman <maciej.wieczor-retman@...el.com>, linux-kernel@...r.kernel.org
Subject: [PATCH v7 07/15] x86/mm: Physical address comparisons in fill_p*d/pte

From: Maciej Wieczor-Retman <maciej.wieczor-retman@...el.com>

Calculating page offset returns a pointer without a tag. When comparing
the calculated offset to a tagged page pointer an error is raised
because they are not equal.

Change pointer comparisons to physical address comparisons as to avoid
issues with tagged pointers that pointer arithmetic would create. Open
code pte_offset_kernel(), pmd_offset(), pud_offset() and p4d_offset().
Because one parameter is always zero and the rest of the function
insides are enclosed inside __va(), removing that layer lowers the
complexity of final assembly.

Signed-off-by: Maciej Wieczor-Retman <maciej.wieczor-retman@...el.com>
---
Changelog v7:
- Swap ternary operator outcomes in fill_p4d since the order was
  incorrect.

Changelog v2:
- Open code *_offset() to avoid it's internal __va().

 arch/x86/mm/init_64.c | 11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)

diff --git a/arch/x86/mm/init_64.c b/arch/x86/mm/init_64.c
index 0e4270e20fad..ae173f8c2aa7 100644
--- a/arch/x86/mm/init_64.c
+++ b/arch/x86/mm/init_64.c
@@ -269,7 +269,10 @@ static p4d_t *fill_p4d(pgd_t *pgd, unsigned long vaddr)
 	if (pgd_none(*pgd)) {
 		p4d_t *p4d = (p4d_t *)spp_getpage();
 		pgd_populate(&init_mm, pgd, p4d);
-		if (p4d != p4d_offset(pgd, 0))
+
+		if (__pa(p4d) != (pgtable_l5_enabled() ?
+				  (unsigned long)pgd_val(*pgd) & PTE_PFN_MASK :
+				  __pa(pgd)))
 			printk(KERN_ERR "PAGETABLE BUG #00! %p <-> %p\n",
 			       p4d, p4d_offset(pgd, 0));
 	}
@@ -281,7 +284,7 @@ static pud_t *fill_pud(p4d_t *p4d, unsigned long vaddr)
 	if (p4d_none(*p4d)) {
 		pud_t *pud = (pud_t *)spp_getpage();
 		p4d_populate(&init_mm, p4d, pud);
-		if (pud != pud_offset(p4d, 0))
+		if (__pa(pud) != (p4d_val(*p4d) & p4d_pfn_mask(*p4d)))
 			printk(KERN_ERR "PAGETABLE BUG #01! %p <-> %p\n",
 			       pud, pud_offset(p4d, 0));
 	}
@@ -293,7 +296,7 @@ static pmd_t *fill_pmd(pud_t *pud, unsigned long vaddr)
 	if (pud_none(*pud)) {
 		pmd_t *pmd = (pmd_t *) spp_getpage();
 		pud_populate(&init_mm, pud, pmd);
-		if (pmd != pmd_offset(pud, 0))
+		if (__pa(pmd) != (pud_val(*pud) & pud_pfn_mask(*pud)))
 			printk(KERN_ERR "PAGETABLE BUG #02! %p <-> %p\n",
 			       pmd, pmd_offset(pud, 0));
 	}
@@ -305,7 +308,7 @@ static pte_t *fill_pte(pmd_t *pmd, unsigned long vaddr)
 	if (pmd_none(*pmd)) {
 		pte_t *pte = (pte_t *) spp_getpage();
 		pmd_populate_kernel(&init_mm, pmd, pte);
-		if (pte != pte_offset_kernel(pmd, 0))
+		if (__pa(pte) != (pmd_val(*pmd) & pmd_pfn_mask(*pmd)))
 			printk(KERN_ERR "PAGETABLE BUG #03!\n");
 	}
 	return pte_offset_kernel(pmd, vaddr);
-- 
2.52.0