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Subject[PATCH 5.4 178/309] btrfs: Correctly handle empty trees in find_first_clear_extent_bit
Date
From: Nikolay Borisov <nborisov@suse.com>

commit 5750c37523a2c8cbb450b9ef31e21c2ba876b05e upstream.

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: 45bfcfc168f8 ("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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
fs/btrfs/extent_io.c | 32 ++++++++++++++++++--------------
fs/btrfs/tests/extent-io-tests.c | 9 +++++++++
2 files changed, 27 insertions(+), 14 deletions(-)

--- a/fs/btrfs/extent_io.c
+++ b/fs/btrfs/extent_io.c
@@ -1583,21 +1583,25 @@ void find_first_clear_extent_bit(struct
/* 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
--- a/fs/btrfs/tests/extent-io-tests.c
+++ b/fs/btrfs/tests/extent-io-tests.c
@@ -441,8 +441,17 @@ static int test_find_first_clear_extent_
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

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