linux-kernel - [RFC PATCH v3 05/16] powerpc/mm: Fix __find_linux

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Date: Sun, 26 May 2024 11:22:25 +0200
From: Christophe Leroy <christophe.leroy@...roup.eu>
To: Andrew Morton <akpm@...ux-foundation.org>,
	Jason Gunthorpe <jgg@...dia.com>,
	Peter Xu <peterx@...hat.com>,
	Oscar Salvador <osalvador@...e.de>,
	Michael Ellerman <mpe@...erman.id.au>,
	Nicholas Piggin <npiggin@...il.com>
Cc: Christophe Leroy <christophe.leroy@...roup.eu>,
	linux-kernel@...r.kernel.org,
	linux-mm@...ck.org,
	linuxppc-dev@...ts.ozlabs.org
Subject: [RFC PATCH v3 05/16] powerpc/mm: Fix __find_linux_pte() on 32 bits with PMD leaf entries

Building on 32 bits with pmd_leaf() not returning always false leads
to the following error:

  CC      arch/powerpc/mm/pgtable.o
arch/powerpc/mm/pgtable.c: In function '__find_linux_pte':
arch/powerpc/mm/pgtable.c:506:1: error: function may return address of local variable [-Werror=return-local-addr]
  506 | }
      | ^
arch/powerpc/mm/pgtable.c:394:15: note: declared here
  394 |         pud_t pud, *pudp;
      |               ^~~
arch/powerpc/mm/pgtable.c:394:15: note: declared here

This is due to pmd_offset() being a no-op in that case.

So rework it for powerpc/32 so that pXd_offset() are used on real
pointers and not on on-stack copies.

Signed-off-by: Christophe Leroy <christophe.leroy@...roup.eu>
---
v3: Removed p4dp and pudp locals for PPC32 and add a comment.
---
 arch/powerpc/mm/pgtable.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/arch/powerpc/mm/pgtable.c b/arch/powerpc/mm/pgtable.c
index 9e7ba9c3851f..10adef5967a3 100644
--- a/arch/powerpc/mm/pgtable.c
+++ b/arch/powerpc/mm/pgtable.c
@@ -382,8 +382,10 @@ pte_t *__find_linux_pte(pgd_t *pgdir, unsigned long ea,
 			bool *is_thp, unsigned *hpage_shift)
 {
 	pgd_t *pgdp;
+#ifdef CONFIG_PPC64
 	p4d_t p4d, *p4dp;
 	pud_t pud, *pudp;
+#endif
 	pmd_t pmd, *pmdp;
 	pte_t *ret_pte;
 	hugepd_t *hpdp = NULL;
@@ -401,8 +403,12 @@ pte_t *__find_linux_pte(pgd_t *pgdir, unsigned long ea,
 	 * page fault or a page unmap. The return pte_t * is still not
 	 * stable. So should be checked there for above conditions.
 	 * Top level is an exception because it is folded into p4d.
+	 *
+	 * On PPC32, P4D/PUD/PMD are folded into PGD so go straight to
+	 * PMD level.
 	 */
 	pgdp = pgdir + pgd_index(ea);
+#ifdef CONFIG_PPC64
 	p4dp = p4d_offset(pgdp, ea);
 	p4d  = READ_ONCE(*p4dp);
 	pdshift = P4D_SHIFT;
@@ -444,6 +450,9 @@ pte_t *__find_linux_pte(pgd_t *pgdir, unsigned long ea,
 
 	pdshift = PMD_SHIFT;
 	pmdp = pmd_offset(&pud, ea);
+#else
+	pmdp = pmd_offset(pud_offset(p4d_offset(pgdp, ea), ea), ea);
+#endif
 	pmd  = READ_ONCE(*pmdp);
 
 	/*
-- 
2.44.0