shmem: use monotonic time for i_generation

get_seconds() is deprecated because it will lead to a 32-bit overflow in
2038 or 2106.  We don't need the i_generation to be strictly monotonic
anyway, and other file systems like ext4 and xfs just use prandom_u32(),
so let's use the same one here.

If this is considered too slow, we could also use ktime_get_seconds() or
ktime_get_real_seconds() to keep the previous behavior.  Both of these
return a time64_t and are not deprecated, but only return a unique value
once per second, and are predictable.

Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Arnd Bergmann <[email protected]>
Reviewed-by: Andrew Morton <[email protected]>
Cc: Hugh Dickins <[email protected]>
Cc: Mike Kravetz <[email protected]>
Cc: "Kirill A. Shutemov" <[email protected]>
Cc: Michal Hocko <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

mm/shmem.c

@@ -29,6 +29,7 @@
 #include <linux/pagemap.h>
 #include <linux/file.h>
 #include <linux/mm.h>
+#include <linux/random.h>
 #include <linux/sched/signal.h>
 #include <linux/export.h>
 #include <linux/swap.h>
@@ -2188,7 +2189,7 @@ static struct inode *shmem_get_inode(struct super_block *sb, const struct inode
 		inode_init_owner(inode, dir, mode);
 		inode->i_blocks = 0;
 		inode->i_atime = inode->i_mtime = inode->i_ctime = current_time(inode);
-		inode->i_generation = get_seconds();
+		inode->i_generation = prandom_u32();
 		info = SHMEM_I(inode);
 		memset(info, 0, (char *)inode - (char *)info);
 		spin_lock_init(&info->lock);