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

Qu Wenruo
2020-07-03T05:19:29Z

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On 2020/7/3 上午7:46, David Sterba wrote:
> 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.
>

Thanks for the merge and dropping the unneeded check patch.

All the modification looks good to me.

Just a small nitpick for commit a561defc34aa ("btrfs: don't allocate
anonymous block device for user invisible roots"), there is an
unnecessary new line after "[CAUSE]".

Thanks for your daily work of maintaining btrfs,
Qu

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