linux-kernel - [PATCH RFC 03/11] x86/mm: Add lookup_pgtable_in

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Message-ID: <20250313-asi-page-alloc-v1-3-04972e046cea@google.com>
Date: Thu, 13 Mar 2025 18:11:22 +0000
From: Brendan Jackman <jackmanb@...gle.com>
To: Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>, 
	Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org, 
	Andrew Morton <akpm@...ux-foundation.org>, David Rientjes <rientjes@...gle.com>, 
	Vlastimil Babka <vbabka@...e.cz>, David Hildenbrand <david@...hat.com>
Cc: linux-kernel@...r.kernel.org, linux-mm@...ck.org, 
	Mike Rapoport <rppt@...nel.org>, Junaid Shahid <junaids@...gle.com>, Reiji Watanabe <reijiw@...gle.com>, 
	Patrick Bellasi <derkling@...gle.com>, Brendan Jackman <jackmanb@...gle.com>, 
	Yosry Ahmed <yosry.ahmed@...ux.dev>
Subject: [PATCH RFC 03/11] x86/mm: Add lookup_pgtable_in_pgd()

This is the same thing as lookup_address_in_pgd(), but it returns the
pagetable unconditionally instead of returning NULL when the pagetable
is none. This will be used for looking up and modifying pages that are
*_none() in order to map memory into the ASI restricted address space.

For a [PATCH], if this logic is needed, the surrounding code should
probably first be somewhat refactored. It now looks pretty repetitive,
and it's confusing that lookup_address_in_pgd() returns NULL when
pmd_none() but note when pte_none(). For now here's something that
works.

Signed-off-by: Brendan Jackman <jackmanb@...gle.com>
---
 arch/x86/include/asm/pgtable_types.h |  2 ++
 arch/x86/mm/pat/set_memory.c         | 34 ++++++++++++++++++++++++++++++++++
 2 files changed, 36 insertions(+)

diff --git a/arch/x86/include/asm/pgtable_types.h b/arch/x86/include/asm/pgtable_types.h
index 4b804531b03c3ce5cc48f0a75cb75d58b985777a..e09b509e525534f31c986d705e07b25dd9c04cb7 100644
--- a/arch/x86/include/asm/pgtable_types.h
+++ b/arch/x86/include/asm/pgtable_types.h
@@ -572,6 +572,8 @@ extern pte_t *lookup_address_in_pgd(pgd_t *pgd, unsigned long address,
 				    unsigned int *level);
 pte_t *lookup_address_in_pgd_attr(pgd_t *pgd, unsigned long address,
 				  unsigned int *level, bool *nx, bool *rw);
+extern pte_t *lookup_pgtable_in_pgd(pgd_t *pgd, unsigned long address,
+				    unsigned int *level);
 extern pmd_t *lookup_pmd_address(unsigned long address);
 extern phys_addr_t slow_virt_to_phys(void *__address);
 extern int __init kernel_map_pages_in_pgd(pgd_t *pgd, u64 pfn,
diff --git a/arch/x86/mm/pat/set_memory.c b/arch/x86/mm/pat/set_memory.c
index ef4514d64c0524e5854fa106e3f37ff1e1ba10a2..d066bf2c9e93e126757bd32a7a666db89b2488b6 100644
--- a/arch/x86/mm/pat/set_memory.c
+++ b/arch/x86/mm/pat/set_memory.c
@@ -658,6 +658,40 @@ static inline pgprot_t verify_rwx(pgprot_t old, pgprot_t new, unsigned long star
 	return new;
 }
 
+/*
+ * Lookup the page table entry for a virtual address in a specific pgd. Return
+ * the pointer to the entry, without implying that any mapping actually exists
+ * (the returned value may be zero).
+ */
+pte_t *lookup_pgtable_in_pgd(pgd_t *pgd, unsigned long address, unsigned int *level)
+{
+	p4d_t *p4d;
+	pud_t *pud;
+	pmd_t *pmd;
+
+	*level = PG_LEVEL_256T;
+	if (pgd_none(*pgd))
+		return (pte_t *)pgd;
+
+	*level = PG_LEVEL_512G;
+	p4d = p4d_offset(pgd, address);
+	if (p4d_none(*p4d) || p4d_leaf(*p4d) || !p4d_present(*p4d))
+		return (pte_t *)p4d;
+
+	*level = PG_LEVEL_1G;
+	pud = pud_offset(p4d, address);
+	if (pud_none(*pud) || pud_leaf(*pud) || !pud_present(*pud))
+		return (pte_t *)pud;
+
+	*level = PG_LEVEL_2M;
+	pmd = pmd_offset(pud, address);
+	if (pmd_none(*pmd) || pmd_leaf(*pmd) || !pmd_present(*pmd))
+		return (pte_t *)pmd;
+
+	*level = PG_LEVEL_4K;
+	return pte_offset_kernel(pmd, address);
+}
+
 /*
  * Lookup the page table entry for a virtual address in a specific pgd.
  * Return a pointer to the entry (or NULL if the entry does not exist),

-- 
2.49.0.rc1.451.g8f38331e32-goog