[PATCH 5.4 25/27] btrfs: qgroup: dont commit transaction when we already hold the handle

Greg Kroah-Hartman
2021-08-13T15:15:53Z

From: Qu Wenruo <wqu@suse.com>

commit 6f23277a49e68f8a9355385c846939ad0b1261e7 upstream

[BUG]
When running the following script, btrfs will trigger an ASSERT():

#/bin/bash
mkfs.btrfs -f $dev
mount $dev $mnt
xfs_io -f -c "pwrite 0 1G" $mnt/file
sync
btrfs quota enable $mnt
btrfs quota rescan -w $mnt

# Manually set the limit below current usage
btrfs qgroup limit 512M $mnt $mnt

# Crash happens
touch $mnt/file

The dmesg looks like this:

assertion failed: refcount_read(&trans->use_count) == 1, in fs/btrfs/transaction.c:2022
------------[ cut here ]------------
kernel BUG at fs/btrfs/ctree.h:3230!
invalid opcode: 0000 [#1] SMP PTI
RIP: 0010:assertfail.constprop.0+0x18/0x1a [btrfs]
btrfs_commit_transaction.cold+0x11/0x5d [btrfs]
try_flush_qgroup+0x67/0x100 [btrfs]
__btrfs_qgroup_reserve_meta+0x3a/0x60 [btrfs]
btrfs_delayed_update_inode+0xaa/0x350 [btrfs]
btrfs_update_inode+0x9d/0x110 [btrfs]
btrfs_dirty_inode+0x5d/0xd0 [btrfs]
touch_atime+0xb5/0x100
iterate_dir+0xf1/0x1b0
__x64_sys_getdents64+0x78/0x110
do_syscall_64+0x33/0x80
entry_SYSCALL_64_after_hwframe+0x44/0xa9
RIP: 0033:0x7fb5afe588db

[CAUSE]
In try_flush_qgroup(), we assume we don't hold a transaction handle at
all. This is true for data reservation and mostly true for metadata.
Since data space reservation always happens before we start a
transaction, and for most metadata operation we reserve space in
start_transaction().

But there is an exception, btrfs_delayed_inode_reserve_metadata().
It holds a transaction handle, while still trying to reserve extra
metadata space.

When we hit EDQUOT inside btrfs_delayed_inode_reserve_metadata(), we
will join current transaction and commit, while we still have
transaction handle from qgroup code.

[FIX]
Let's check current->journal before we join the transaction.

If current->journal is unset or BTRFS_SEND_TRANS_STUB, it means
we are not holding a transaction, thus are able to join and then commit
transaction.

If current->journal is a valid transaction handle, we avoid committing
transaction and just end it

This is less effective than committing current transaction, as it won't
free metadata reserved space, but we may still free some data space
before new data writes.

Bugzilla: https://bugzilla.suse.com/show_bug.cgi?id=1178634
Fixes: c53e9653605d ("btrfs: qgroup: try to flush qgroup space when we get -EDQUOT")
Reviewed-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>
Signed-off-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
fs/btrfs/qgroup.c | 20 +++++++++++++++++++-
1 file changed, 19 insertions(+), 1 deletion(-)

--- a/fs/btrfs/qgroup.c
+++ b/fs/btrfs/qgroup.c
@@ -3502,6 +3502,7 @@ static int try_flush_qgroup(struct btrfs
{
struct btrfs_trans_handle *trans;
int ret;
+ bool can_commit = true;

/*
* We don't want to run flush again and again, so if there is a running
@@ -3513,6 +3514,20 @@ static int try_flush_qgroup(struct btrfs
return 0;
}

+ /*
+ * If current process holds a transaction, we shouldn't flush, as we
+ * assume all space reservation happens before a transaction handle is
+ * held.
+ *
+ * But there are cases like btrfs_delayed_item_reserve_metadata() where
+ * we try to reserve space with one transction handle already held.
+ * In that case we can't commit transaction, but at least try to end it
+ * and hope the started data writes can free some space.
+ */
+ if (current->journal_info &&
+ current->journal_info != BTRFS_SEND_TRANS_STUB)
+ can_commit = false;
+
ret = btrfs_start_delalloc_snapshot(root);
if (ret < 0)
goto out;
@@ -3524,7 +3539,10 @@ static int try_flush_qgroup(struct btrfs
goto out;
}

- ret = btrfs_commit_transaction(trans);
+ if (can_commit)
+ ret = btrfs_commit_transaction(trans);
+ else
+ ret = btrfs_end_transaction(trans);
out:
clear_bit(BTRFS_ROOT_QGROUP_FLUSHING, &root->state);
wake_up(&root->qgroup_flush_wait);