Re: [PATCH v2] btrfs: qgroup: fix sleep from invalid context bug in update_qgroup_limit_item()

[Qu Wenruo <quwenruo.btrfs@gmx.com>]

On 2022/11/11 17:02, ChenXiaoSong wrote:
> Syzkaller reported BUG as follows:
> 
>    BUG: sleeping function called from invalid context at
>         include/linux/sched/mm.h:274
>    Call Trace:
>     <TASK>
>     dump_stack_lvl+0xcd/0x134
>     __might_resched.cold+0x222/0x26b
>     kmem_cache_alloc+0x2e7/0x3c0
>     update_qgroup_limit_item+0xe1/0x390
>     btrfs_qgroup_inherit+0x147b/0x1ee0
>     create_subvol+0x4eb/0x1710
>     btrfs_mksubvol+0xfe5/0x13f0
>     __btrfs_ioctl_snap_create+0x2b0/0x430
>     btrfs_ioctl_snap_create_v2+0x25a/0x520
>     btrfs_ioctl+0x2a1c/0x5ce0
>     __x64_sys_ioctl+0x193/0x200
>     do_syscall_64+0x35/0x80
> 
> Fix this by delaying the limit item updates until unlock the spin lock.

The overall idea is way better now.

But sorry that I didn't immediately find out the best solution at the 
first glance.

In fact, your v2 path just lets me remember what is the correct way to 
handle such situation.

> 
> Signed-off-by: ChenXiaoSong <chenxiaosong2@huawei.com>
> ---
>   fs/btrfs/qgroup.c | 18 +++++++++---------
>   1 file changed, 9 insertions(+), 9 deletions(-)
> 
> diff --git a/fs/btrfs/qgroup.c b/fs/btrfs/qgroup.c
> index 9334c3157c22..2792d63c0da4 100644
> --- a/fs/btrfs/qgroup.c
> +++ b/fs/btrfs/qgroup.c
[...]
> @@ -2985,6 +2978,7 @@ int btrfs_qgroup_inherit(struct btrfs_trans_handle *trans, u64 srcid,
>   		dstgroup->max_excl = srcgroup->max_excl;
>   		dstgroup->rsv_rfer = srcgroup->rsv_rfer;
>   		dstgroup->rsv_excl = srcgroup->rsv_excl;
> +		update_limit = true;
>   
>   		qgroup_dirty(fs_info, dstgroup);
>   		qgroup_dirty(fs_info, srcgroup);

You caught the "if (srcid)" branch, which also changed the 
limit/rfer/excl numbers of the destination qgroup, but didn't call 
update_qgroup_limit_item().


But if you check the function qgroup_dirty() a little deeper, you can 
find out that, qgroup_dirty() will move the target qgroup into 
fs_info->dirty_qgroups list.

And later at btrfs_run_qgroups() (which is called during 
btrfs_commit_transaction(), and also after create_pending_snapshots()), 
we will update the quota tree to reflect the result.

So this means, all you need is just call qgroup_dirty() on @dstqgroup, 
and call it a day.
Everything else will be properly handled.

Sorry I didn't notice this earlier...

Thanks,
Qu

> @@ -3053,6 +3047,12 @@ int btrfs_qgroup_inherit(struct btrfs_trans_handle *trans, u64 srcid,
>   
>   unlock:
>   	spin_unlock(&fs_info->qgroup_lock);
> +	if (update_limit && update_qgroup_limit_item(trans, dstgroup)) {
> +		qgroup_mark_inconsistent(fs_info);
> +		btrfs_info(fs_info,
> +			   "unable to update quota limit for %llu",
> +			   dstgroup->qgroupid);
> +	}
>   	if (!ret)
>   		ret = btrfs_sysfs_add_one_qgroup(fs_info, dstgroup);
>   out: