Re: [PATCH] btrfs: fix lockdep warning chunk_mutex vs device_list_mutex

[David Sterba <dsterba@suse.cz>]

On Mon, May 18, 2020 at 07:07:21PM +0800, Anand Jain wrote:
> 
> 
> On 16/5/20 11:43 am, Anand Jain wrote:
> > On 16/5/20 1:40 am, David Sterba wrote:
> >> On Thu, May 14, 2020 at 03:46:59AM +0800, Anand Jain wrote:
> >>> A full list of tests just started.
> >>>
> >>>   fs/btrfs/volumes.c | 8 +++++---
> >>>   1 file changed, 5 insertions(+), 3 deletions(-)
> >>>
> >>> diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
> >>> index 60ab41c12e50..ebc8565d0f73 100644
> >>> --- a/fs/btrfs/volumes.c
> >>> +++ b/fs/btrfs/volumes.c
> >>> @@ -984,7 +984,6 @@ static struct btrfs_fs_devices 
> >>> *clone_fs_devices(struct btrfs_fs_devices *orig)
> >>>       if (IS_ERR(fs_devices))
> >>>           return fs_devices;
> >>
> >> So now here's the device_list_mutex taken by a caller but inside
> >> clone_fs_devices there's
> >>
> >>     fs_devices = alloc_fs_devices(orig->fsid, NULL);
> >>
> >> just before this line and it does a GFP_KERNEL allocation.
> > 
> > Oh right the allocations. Its not just about the other locks
> > as I thought before.
> > 
> > There are two ways to fix.
> >    Use GFP_NOFS
> >     I am not yet sure if it not possible. There were some previous
> >     work on the GFP flags. I need to review them. or,
> >    Move the allocation outside the locks.
> > 
> >   Looking into both of these choices.
> > 
> 
> 
> Nack. On this patch.
> 
> In general GFP_KERNEL is preferred over GFP_NOFS. For example.
> 
> --------
> 6165572c btrfs: use GFP_KERNEL in btrfs_init_dev_replace_tgtdev
> cc8385b5 btrfs: preallocate radix tree node for readahead
> 78f2c9e6 btrfs: device add and remove: use GFP_KERNEL
> --------
> 
> And there are quite a lot of GFP_KERNEL allocation along the
> path leading to clone_fs_devices().

For that we have the scoped NOFS, using the
memalloc_nofs_save/memalloc_nofs_restore. So if this is set before/after
the device_list_mutex is taken when calling the cloning then it's safe
with any potential GFP_KERNEL allocations on the way.