Re: [PATCH 3/4] btrfs: preallocate anon_dev for subvolume and snapshot creation

[David Sterba <dsterba@suse.cz>]

On Thu, Jul 02, 2020 at 06:08:21PM +0200, David Sterba wrote:
> On Thu, Jul 02, 2020 at 07:56:57AM +0800, Qu Wenruo wrote:
> > On 2020/7/2 上午1:39, David Sterba wrote:
> > > On Wed, Jul 01, 2020 at 11:25:27AM +0800, Qu Wenruo wrote:
> > > Adding the anon_dev argument to btrfs_get_fs_root is wrong and I have
> > > never suggested that. What I meant is to put the actual id allocation
> > > to the callers where the subvolume is created, ie only 2 places.
> > 
> > You mean to extract btrfs_init_fs_root() out of btrfs_get_fs_root()?
> > 
> > That looks a little risky and I can't find any good solution to make it
> > more elegant than the current one.
> 
> I spent more time reading through the get-fs-root functions and the main
> problem is that btrfs_get_fs_root is doing several things, and it makes
> a lot of code simple, I certainly want to keep it that way.
> 
> The idea was to pre-insert the new root (similar to the root item
> insertion, btrfs_insert_root) and not letting btrfs_get_fs_root call to
> btrfs_init_fs_info where the anon_bdev allocation happens for all the
> other non-ioctl cases.
> 
> Which could be done by factoring out btrfs_init_fs_root from
> btrfs_get_fs_root. This would allow to extend only btrfs_init_fs_root
> arguments with the anon_bdev, and keep btrfs_get_fs_root intact.
> So this is splitting the API from the end.
> 
> What you originally proposed is a split from the begnning, ie. add a
> common implementation for existing and new and provide btrfs_get_fs_root
> and btrfs_get_new_fs_root that would hide the additional parameters.
> 
> Both ways are IMO valid but I thought it would be easier to pass the
> anon bdev inside ioctl callbacks. The problem that makes my proposal
> less appealing is that btrfs_read_tree_root gets called earlier than
> I'd like so factoring everything after btrfs_init_fs_root would not be
> so straightforward.
> 
> In conclusion, your proposal is better and I'm going to merge it.
> 
> > Although I would definitely remove the "__" prefix as we shouldn't add
> > such prefix anymore.
> 
> Yeah with the small naming fixups.

It's in for-next-20200703. I've updated the changelogs to reflect what
we found during debugging the issue, the __ function renamed to
btrfs_get_root_ref and some function comments added. All patches
reordered and tagged for stable though the preallocation is not within
the size limit.