From 789cc627f6c29059e5747101352a1b28ca8d62a6 Mon Sep 17 00:00:00 2001
From: Donald Buczek <buczek@molgen.mpg.de>
Date: Wed, 5 Jun 2024 12:13:03 +0200
Subject: [PATCH] Revert "mm/sparsemem: fix race in accessing
 memory_section->usage"

This reverts commit b448de2459b6d62a53892487ab18b7d823ff0529.

For some reasons we don't understand, b448de2459b6d6 ("mm/sparsemem: fix
race in accessing memory_section->usage") prevents us from compiling the
proprietary Nvidia Unix driver with versions 510.108.03 and 535.104.05
in the 5.15 series.  No problems with the 6.5 series, although the patch
is included there as well

    make -j $(nproc) CC=gcc SYSSRC=/scratch/local/linux clean && make -j $(nproc) CC=gcc SYSSRC=/scratch/local/linux V=2 modules
    [...]
    MODPOST /scratch/local/bee-buczek/nvidia/test_535-104-05/kernel/Module.symvers - due to target missing
    ERROR: modpost: GPL-incompatible module nvidia.ko uses GPL-only symbol 'rcu_read_unlock_strict'
    make[2]: *** [scripts/Makefile.modpost:133: /scratch/local/bee-buczek/nvidia/test_535-104-05/kernel/Module.symvers] Error 1
    make[2]: *** Deleting file '/scratch/local/bee-buczek/nvidia/test_535-104-05/kernel/Module.symvers'
    make[1]: *** [Makefile:1826: modules] Error 2
    make[1]: Leaving directory '/scratch/local/linux'
    make: *** [Makefile:82: modules] Error 2
---
 include/linux/mmzone.h | 14 +++-----------
 mm/sparse.c            | 17 ++++++++---------
 2 files changed, 11 insertions(+), 20 deletions(-)

diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
index 8b8349ffa1cd9..9e1485083398a 100644
--- a/include/linux/mmzone.h
+++ b/include/linux/mmzone.h
@@ -1287,7 +1287,6 @@ static inline unsigned long section_nr_to_pfn(unsigned long sec)
 #define SUBSECTION_ALIGN_DOWN(pfn) ((pfn) & PAGE_SUBSECTION_MASK)
 
 struct mem_section_usage {
-	struct rcu_head rcu;
 #ifdef CONFIG_SPARSEMEM_VMEMMAP
 	DECLARE_BITMAP(subsection_map, SUBSECTIONS_PER_SECTION);
 #endif
@@ -1458,7 +1457,7 @@ static inline int pfn_section_valid(struct mem_section *ms, unsigned long pfn)
 {
 	int idx = subsection_map_index(pfn);
 
-	return test_bit(idx, READ_ONCE(ms->usage)->subsection_map);
+	return test_bit(idx, ms->usage->subsection_map);
 }
 #else
 static inline int pfn_section_valid(struct mem_section *ms, unsigned long pfn)
@@ -1482,7 +1481,6 @@ static inline int pfn_section_valid(struct mem_section *ms, unsigned long pfn)
 static inline int pfn_valid(unsigned long pfn)
 {
 	struct mem_section *ms;
-	int ret;
 
 	/*
 	 * Ensure the upper PAGE_SHIFT bits are clear in the
@@ -1496,19 +1494,13 @@ static inline int pfn_valid(unsigned long pfn)
 	if (pfn_to_section_nr(pfn) >= NR_MEM_SECTIONS)
 		return 0;
 	ms = __pfn_to_section(pfn);
-	rcu_read_lock();
-	if (!valid_section(ms)) {
-		rcu_read_unlock();
+	if (!valid_section(ms))
 		return 0;
-	}
 	/*
 	 * Traditionally early sections always returned pfn_valid() for
 	 * the entire section-sized span.
 	 */
-	ret = early_section(ms) || pfn_section_valid(ms, pfn);
-	rcu_read_unlock();
-
-	return ret;
+	return early_section(ms) || pfn_section_valid(ms, pfn);
 }
 #endif
 
diff --git a/mm/sparse.c b/mm/sparse.c
index 27092badd15bd..120bc8ea5293e 100644
--- a/mm/sparse.c
+++ b/mm/sparse.c
@@ -789,13 +789,6 @@ static void section_deactivate(unsigned long pfn, unsigned long nr_pages,
 	if (empty) {
 		unsigned long section_nr = pfn_to_section_nr(pfn);
 
-		/*
-		 * Mark the section invalid so that valid_section()
-		 * return false. This prevents code from dereferencing
-		 * ms->usage array.
-		 */
-		ms->section_mem_map &= ~SECTION_HAS_MEM_MAP;
-
 		/*
 		 * When removing an early section, the usage map is kept (as the
 		 * usage maps of other sections fall into the same page). It
@@ -804,10 +797,16 @@ static void section_deactivate(unsigned long pfn, unsigned long nr_pages,
 		 * was allocated during boot.
 		 */
 		if (!PageReserved(virt_to_page(ms->usage))) {
-			kfree_rcu(ms->usage, rcu);
-			WRITE_ONCE(ms->usage, NULL);
+			kfree(ms->usage);
+			ms->usage = NULL;
 		}
 		memmap = sparse_decode_mem_map(ms->section_mem_map, section_nr);
+		/*
+		 * Mark the section invalid so that valid_section()
+		 * return false. This prevents code from dereferencing
+		 * ms->usage array.
+		 */
+		ms->section_mem_map &= ~SECTION_HAS_MEM_MAP;
 	}
 
 	/*