mm: move drivers/char/mem.c to mm/char-mem.c

The memory character driver implements several mm-specific features and is
always compiled into the kernel, so move it to mm/ where it belongs.

Among other things the driver implements /dev/mem which provides raw access
to physical memory, and /dev/zero which either allows mapping of a shmem
region (if mapped with MAP_SHARED) or, uniquely, anonymous memory (if
mapped MAP_PRIVATE).

This change lays the foundations to allow MAP_PRIVATE-/dev/zero to be
mapped precisely the same as anonymous memory is mapped as currently it is
an edge case within mm.

Also update a couple of comments that reference 'drivers/char/mem.c' to
reference 'mm/char-mem.c'.

Link: https://lore.kernel.org/20260926-map-private-dev-zero-v3-1-d4781e84ccfc@kernel.org
Signed-off-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Acked-by: David Hildenbrand (Arm) <david@kernel.org>
Acked-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Liam R. Howlett <liam@infradead.org>
Cc: Vlastimil Babka <vbabka@kernel.org>
Cc: Jann Horn <jannh@google.com>
Cc: Pedro Falcato <pfalcato@suse.de>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Jan Kara <jack@suse.cz>

MAINTAINERS

@@ -17269,7 +17269,7 @@ F:	Documentation/ABI/testing/sysfs-kernel-mm-cma
 F:	Documentation/ABI/testing/sysfs-kernel-mm-numa
 F:	Documentation/admin-guide/mm/
 F:	Documentation/mm/
-F:	drivers/char/mem.c
+F:	mm/char-mem.c
 F:	include/linux/cma.h
 F:	include/linux/dmapool.h
 F:	include/linux/ioremap.h
@@ -17493,7 +17493,7 @@ L:	linux-mm@kvack.org
 S:	Maintained
 W:	http://www.linux-mm.org
 T:	git git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm
-F:	drivers/char/mem.c
+F:	mm/char-mem.c
 F:	include/trace/events/mmap.h
 F:	fs/proc/task_mmu.c
 F:	fs/proc/task_nommu.c

drivers/char/Makefile

@@ -3,7 +3,7 @@
 # Makefile for the kernel character device drivers.
 #
 
-obj-y				+= mem.o random.o
+obj-y				+= random.o
 obj-$(CONFIG_TTY_PRINTK)	+= ttyprintk.o
 obj-y				+= misc.o
 obj-$(CONFIG_TEST_MISC_MINOR)	+= misc_minor_kunit.o

mm/Makefile

@@ -55,7 +55,8 @@ obj-y			:= filemap.o mempool.o oom_kill.o fadvise.o \
 			   mm_init.o percpu.o slab_common.o \
 			   compaction.o show_mem.o \
 			   interval_tree.o list_lru.o workingset.o \
-			   debug.o gup.o mmap_lock.o vma_init.o $(mmu-y)
+			   debug.o gup.o mmap_lock.o vma_init.o char-mem.o \
+			   $(mmu-y)
 
 # Give 'page_alloc' its own module-parameter namespace
 page-alloc-y := page_alloc.o

mm/char-mem.c

@@ -1,6 +1,6 @@
 // SPDX-License-Identifier: GPL-2.0
 /*
- *  linux/drivers/char/mem.c
+ *  mm/char-mem.c
  *
  *  Copyright (C) 1991, 1992  Linus Torvalds
  *

mm/shmem.c

@@ -2984,7 +2984,7 @@ unsigned long shmem_get_unmapped_area(struct file *file,
 			sb = file_inode(file)->i_sb;
 		} else {
 			/*
-			 * Called directly from mm/mmap.c, or drivers/char/mem.c
+			 * Called directly from mm/mmap.c, or mm/char-mem.c
 			 * for "/dev/zero", to create a shared anonymous object.
 			 */
 			if (IS_ERR(shm_mnt))