Revert "mm/sparsemem: fix race in accessing memory_section->usage"

This reverts commit b448de2.

With this patch, the Nvidia kernel drives doesn't compile, so revert
it.

linux-5.15.155 , nvidia 535.104.05 :

    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

Although the reverted patch  ist included in the 6.5 series, the
Nvidia driver compiles okay with 6.5.

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
 

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;
 	}
 
 	/*