btrfs: Correctly handle empty trees in find_first_clear_extent_bit

Raviu reported that running his regular fs_trim segfaulted with the
following backtrace:

[  237.525947] assertion failed: prev, in ../fs/btrfs/extent_io.c:1595
[  237.525984] ------------[ cut here ]------------
[  237.525985] kernel BUG at ../fs/btrfs/ctree.h:3117!
[  237.525992] invalid opcode: 0000 [#1] SMP PTI
[  237.525998] CPU: 4 PID: 4423 Comm: fstrim Tainted: G     U     OE     5.4.14-8-vanilla #1
[  237.526001] Hardware name: ASUSTeK COMPUTER INC.
[  237.526044] RIP: 0010:assfail.constprop.58+0x18/0x1a [btrfs]
[  237.526079] Call Trace:
[  237.526120]  find_first_clear_extent_bit+0x13d/0x150 [btrfs]
[  237.526148]  btrfs_trim_fs+0x211/0x3f0 [btrfs]
[  237.526184]  btrfs_ioctl_fitrim+0x103/0x170 [btrfs]
[  237.526219]  btrfs_ioctl+0x129a/0x2ed0 [btrfs]
[  237.526227]  ? filemap_map_pages+0x190/0x3d0
[  237.526232]  ? do_filp_open+0xaf/0x110
[  237.526238]  ? _copy_to_user+0x22/0x30
[  237.526242]  ? cp_new_stat+0x150/0x180
[  237.526247]  ? do_vfs_ioctl+0xa4/0x640
[  237.526278]  ? btrfs_ioctl_get_supported_features+0x30/0x30 [btrfs]
[  237.526283]  do_vfs_ioctl+0xa4/0x640
[  237.526288]  ? __do_sys_newfstat+0x3c/0x60
[  237.526292]  ksys_ioctl+0x70/0x80
[  237.526297]  __x64_sys_ioctl+0x16/0x20
[  237.526303]  do_syscall_64+0x5a/0x1c0
[  237.526310]  entry_SYSCALL_64_after_hwframe+0x49/0xbe

That was due to btrfs_fs_device::aloc_tree being empty. Initially I
thought this wasn't possible and as a percaution have put the assert in
find_first_clear_extent_bit. Turns out this is indeed possible and could
happen when a file system with SINGLE data/metadata profile has a 2nd
device added. Until balance is run or a new chunk is allocated on this
device it will be completely empty.

In this case find_first_clear_extent_bit should return the full range
[0, -1ULL] and let the caller handle this i.e for trim the end will be
capped at the size of actual device.

Link: https://lore.kernel.org/linux-btrfs/izW2WNyvy1dEDweBICizKnd2KDwDiDyY2EYQr4YCwk7pkuIpthx-JRn65MPBde00ND6V0_Lh8mW0kZwzDiLDv25pUYWxkskWNJnVP0kgdMA=@protonmail.com/
Fixes: 45bfcfc1 ("btrfs: Implement find_first_clear_extent_bit")
CC: stable@vger.kernel.org # 5.2+
Signed-off-by: Nikolay Borisov <nborisov@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/extent_io.c

@@ -1593,21 +1593,25 @@ void find_first_clear_extent_bit(struct extent_io_tree *tree, u64 start,
 	/* Find first extent with bits cleared */
 	while (1) {
 		node = __etree_search(tree, start, &next, &prev, NULL, NULL);
-		if (!node) {
+		if (!node && !next && !prev) {
+			/*
+			 * Tree is completely empty, send full range and let
+			 * caller deal with it
+			 */
+			*start_ret = 0;
+			*end_ret = -1;
+			goto out;
+		} else if (!node && !next) {
+			/*
+			 * We are past the last allocated chunk, set start at
+			 * the end of the last extent.
+			 */
+			state = rb_entry(prev, struct extent_state, rb_node);
+			*start_ret = state->end + 1;
+			*end_ret = -1;
+			goto out;
+		} else if (!node) {
 			node = next;
-			if (!node) {
-				/*
-				 * We are past the last allocated chunk,
-				 * set start at the end of the last extent. The
-				 * device alloc tree should never be empty so
-				 * prev is always set.
-				 */
-				ASSERT(prev);
-				state = rb_entry(prev, struct extent_state, rb_node);
-				*start_ret = state->end + 1;
-				*end_ret = -1;
-				goto out;
-			}
 		}
 		/*
 		 * At this point 'node' either contains 'start' or start is

fs/btrfs/tests/extent-io-tests.c

@@ -441,8 +441,17 @@ static int test_find_first_clear_extent_bit(void)
 	int ret = -EINVAL;
 
 	test_msg("running find_first_clear_extent_bit test");
+
 	extent_io_tree_init(NULL, &tree, IO_TREE_SELFTEST, NULL);
 
+	/* Test correct handling of empty tree */
+	find_first_clear_extent_bit(&tree, 0, &start, &end, CHUNK_TRIMMED);
+	if (start != 0 || end != -1) {
+		test_err(
+	"error getting a range from completely empty tree: start %llu end %llu",
+			 start, end);
+		goto out;
+	}
 	/*
 	 * Set 1M-4M alloc/discard and 32M-64M thus leaving a hole between
 	 * 4M-32M