ipc: shm: restore MADV_REMOVE functionality on shared memory segments

Commit 17cf28af ("mm/fs: remove truncate_range") removed the
truncate_range inode operation in favour of the fallocate file
operation.

When using SYSV IPC shared memory segments, calling madvise with the
MADV_REMOVE advice on an area of shared memory will attempt to invoke
the .fallocate function for the shm_file_operations, which is NULL and
therefore returns -EOPNOTSUPP to userspace.  The previous behaviour
would inherit the inode_operations from the underlying tmpfs file and
invoke truncate_range there.

This patch restores the previous behaviour by wrapping the underlying
fallocate function in shm_fallocate, as we do for fsync.

[hughd@google.com: use -ENOTSUPP in shm_fallocate()]
Signed-off-by: Will Deacon <will.deacon@arm.com>
Acked-by: Hugh Dickins <hughd@google.com>
Signed-off-by: Hugh Dickins <hughd@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

ipc/shm.c

@@ -393,6 +393,16 @@ static int shm_fsync(struct file *file, loff_t start, loff_t end, int datasync)
 	return sfd->file->f_op->fsync(sfd->file, start, end, datasync);
 }
 
+static long shm_fallocate(struct file *file, int mode, loff_t offset,
+			  loff_t len)
+{
+	struct shm_file_data *sfd = shm_file_data(file);
+
+	if (!sfd->file->f_op->fallocate)
+		return -EOPNOTSUPP;
+	return sfd->file->f_op->fallocate(file, mode, offset, len);
+}
+
 static unsigned long shm_get_unmapped_area(struct file *file,
 	unsigned long addr, unsigned long len, unsigned long pgoff,
 	unsigned long flags)
@@ -410,6 +420,7 @@ static const struct file_operations shm_file_operations = {
 	.get_unmapped_area	= shm_get_unmapped_area,
 #endif
 	.llseek		= noop_llseek,
+	.fallocate	= shm_fallocate,
 };
 
 static const struct file_operations shm_file_operations_huge = {
@@ -418,6 +429,7 @@ static const struct file_operations shm_file_operations_huge = {
 	.release	= shm_release,
 	.get_unmapped_area	= shm_get_unmapped_area,
 	.llseek		= noop_llseek,
+	.fallocate	= shm_fallocate,
 };
 
 int is_file_shm_hugepages(struct file *file)