btrfs: allow defrag to be interruptible

During defrag, at btrfs_defrag_file(), we have this loop that iterates
over a file range in steps no larger than 256K subranges. If the range
is too long, there's no way to interrupt it. So make the loop check in
each iteration if there's signal pending, and if there is, break and
return -AGAIN to userspace.

Before kernel 5.16, we used to allow defrag to be cancelled through a
signal, but that was lost with commit 7b50803 ("btrfs: defrag:
use defrag_one_cluster() to implement btrfs_defrag_file()").

This change adds back the possibility to cancel a defrag with a signal
and keeps the same semantics, returning -EAGAIN to user space (and not
the usually more expected -EINTR).

This is also motivated by a recent bug on 5.16 where defragging a 1 byte
file resulted in iterating from file range 0 to (u64)-1, as hitting the
bug triggered a too long loop, basically requiring one to reboot the
machine, as it was not possible to cancel defrag.

Fixes: 7b50803 ("btrfs: defrag: use defrag_one_cluster() to implement btrfs_defrag_file()")
CC: stable@vger.kernel.org # 5.16
Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

Author: fdmanana

fs/btrfs/ioctl.c

@@ -1546,6 +1546,11 @@ int btrfs_defrag_file(struct inode *inode, struct file_ra_state *ra,
 		/* The cluster size 256K should always be page aligned */
 		BUILD_BUG_ON(!IS_ALIGNED(CLUSTER_SIZE, PAGE_SIZE));
 
+		if (btrfs_defrag_cancelled(fs_info)) {
+			ret = -EAGAIN;
+			break;
+		}
+
 		/* We want the cluster end at page boundary when possible */
 		cluster_end = (((cur >> PAGE_SHIFT) +
 			       (SZ_256K >> PAGE_SHIFT)) << PAGE_SHIFT) - 1;