Make TMPFS_PAGES_MINRESERVED a kernel option

TMPFS_PAGES_MINRESERVED controls how much memory is reserved for the system
and not used by tmpfs.

On very small memory systems, the default value may be too high and this
prevents these small memory systems from using reroot, which is required
for them to install firmware updates.

Submitted by:	Hiroki Mori <yamori813@yahoo.co.jp>
Reviewed by:	mizhka
Differential Revision:	https://reviews.freebsd.org/D13583

sys/conf/options

@@ -640,6 +640,9 @@ NFS_MINDIRATTRTIMO	opt_nfs.h
 NFS_MAXDIRATTRTIMO	opt_nfs.h
 NFS_DEBUG		opt_nfs.h
 
+# TMPFS options
+TMPFS_PAGES_MINRESERVED		opt_tmpfs.h
+
 # For the Bt848/Bt848A/Bt849/Bt878/Bt879 driver
 OVERRIDE_CARD			opt_bktr.h
 OVERRIDE_TUNER			opt_bktr.h

sys/fs/tmpfs/tmpfs.h

@@ -487,7 +487,9 @@ struct tmpfs_dirent *tmpfs_dir_next(struct tmpfs_node *dnode,
  * Amount of memory pages to reserve for the system (e.g., to not use by
  * tmpfs).
  */
+#if !defined(TMPFS_PAGES_MINRESERVED)
 #define TMPFS_PAGES_MINRESERVED		(4 * 1024 * 1024 / PAGE_SIZE)
+#endif
 
 size_t tmpfs_mem_avail(void);
 

sys/fs/tmpfs/tmpfs_vfsops.c

@@ -42,6 +42,9 @@
  * memory-specific data structures and algorithms to automatically
  * allocate and release resources.
  */
+
+#include "opt_tmpfs.h"
+
 #include <sys/cdefs.h>
 __FBSDID("$FreeBSD$");
 

sys/modules/tmpfs/Makefile

@@ -4,6 +4,6 @@
 
 KMOD=	tmpfs
 SRCS=	vnode_if.h \
-	tmpfs_vnops.c tmpfs_fifoops.c tmpfs_vfsops.c tmpfs_subr.c
+	tmpfs_vnops.c tmpfs_fifoops.c tmpfs_vfsops.c tmpfs_subr.c opt_tmpfs.h
 
 .include <bsd.kmod.mk>