powerpc: Define empty_zero_page[] in C

At the time being, empty_zero_page[] is defined in each
platform head.S.

Define it in mm/mem.c instead, and put it in BSS section instead
of the DATA section. Commit 5227cfa7 ("arm64: mm: place
empty_zero_page in bss") explains why it is interesting to have
it in BSS.
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/5838caffa269e0957c5a50cc85477876220298b0.1623063174.git.christophe.leroy@csgroup.eu

arch/powerpc/kernel/head_40x.S

@@ -709,10 +709,6 @@ _GLOBAL(abort)
 	.align	12
 	.globl	sdata
 sdata:
-	.globl	empty_zero_page
-empty_zero_page:
-	.space	4096
-EXPORT_SYMBOL(empty_zero_page)
 	.globl	swapper_pg_dir
 swapper_pg_dir:
 	.space	PGD_TABLE_SIZE

arch/powerpc/kernel/head_44x.S

@@ -1241,10 +1241,6 @@ head_start_common:
 	.align	PAGE_SHIFT
 	.globl	sdata
 sdata:
-	.globl	empty_zero_page
-empty_zero_page:
-	.space	PAGE_SIZE
-EXPORT_SYMBOL(empty_zero_page)
 
 /*
  * To support >32-bit physical addresses, we use an 8KB pgdir.

arch/powerpc/kernel/head_64.S

@@ -1012,8 +1012,3 @@ start_here_common:
 	.globl	swapper_pg_dir
 swapper_pg_dir:
 	.space	PGD_TABLE_SIZE
-
-	.globl	empty_zero_page
-empty_zero_page:
-	.space	PAGE_SIZE
-EXPORT_SYMBOL(empty_zero_page)

arch/powerpc/kernel/head_8xx.S

@@ -795,12 +795,6 @@ _GLOBAL(mmu_pin_tlb)
 	.data
 	.globl	sdata
 sdata:
-	.globl	empty_zero_page
-	.align	PAGE_SHIFT
-empty_zero_page:
-	.space	PAGE_SIZE
-EXPORT_SYMBOL(empty_zero_page)
-
 	.globl	swapper_pg_dir
 swapper_pg_dir:
 	.space	PGD_TABLE_SIZE

arch/powerpc/kernel/head_book3s_32.S

@@ -1216,11 +1216,6 @@ setup_usbgecko_bat:
 	.data
 	.globl	sdata
 sdata:
-	.globl	empty_zero_page
-empty_zero_page:
-	.space	4096
-EXPORT_SYMBOL(empty_zero_page)
-
 	.globl	swapper_pg_dir
 swapper_pg_dir:
 	.space	PGD_TABLE_SIZE

arch/powerpc/kernel/head_fsl_booke.S

@@ -1221,10 +1221,6 @@ _GLOBAL(restore_to_as0)
 	.align	12
 	.globl	sdata
 sdata:
-	.globl	empty_zero_page
-empty_zero_page:
-	.space	4096
-EXPORT_SYMBOL(empty_zero_page)
 	.globl	swapper_pg_dir
 swapper_pg_dir:
 	.space	PGD_TABLE_SIZE

arch/powerpc/mm/mem.c

@@ -28,6 +28,9 @@
 unsigned long long memory_limit;
 bool init_mem_is_free;
 
+unsigned long empty_zero_page[PAGE_SIZE / sizeof(unsigned long)] __page_aligned_bss;
+EXPORT_SYMBOL(empty_zero_page);
+
 pgprot_t phys_mem_access_prot(struct file *file, unsigned long pfn,
 			      unsigned long size, pgprot_t vma_prot)
 {