Re: [PATCH v2] vfs: Pre-allocate superblock in sget_fc() if !test

[Christian Brauner <brauner@xxxxxxxxxx>]

On Mon, Jan 24, 2022 at 11:10:06AM -0500, Waiman Long wrote:
> When the test function is not defined in sget_fc(), we always need
> to allocate a new superblock. So there is no point in acquiring the
> sb_lock twice in this case. Optimize the !test case by pre-allocating
> the superblock first before acquring the lock.
> 
> Signed-off-by: Waiman Long <longman@xxxxxxxxxx>
> ---
>  fs/super.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/fs/super.c b/fs/super.c
> index a6405d44d4ca..6601267f6fe0 100644
> --- a/fs/super.c
> +++ b/fs/super.c
> @@ -520,6 +520,8 @@ struct super_block *sget_fc(struct fs_context *fc,
>  	struct user_namespace *user_ns = fc->global ? &init_user_ns : fc->user_ns;
>  	int err;
>  
> +	if (!test)
> +		goto prealloc;

I mean, now its another goto in there that adds to the confusion.
sget_fc() could _probably_ benefit if it were split into
__sget_test_fc() and __sget_independent_fc() with both ending up being
called from sget_fc() dending on whether or not test is NULL ofc.

This way the test and non-test cases would stop being mangled together
possibly making the logic easier to follow. Would probably also require
to move the common initialization part into a helper callable from both
__sget_independent_fc() and __sget_test_fc() sm like idk:

static struct super_block *__finish_init_super(........)
{
	s->s_fs_info = fc->s_fs_info;
	err = set(s, fc);
	if (err) {
		s->s_fs_info = NULL;
		spin_unlock(&sb_lock);
		destroy_unused_super(s);
		return ERR_PTR(err);
	}
	fc->s_fs_info = NULL;
	s->s_type = fc->fs_type;
	s->s_iflags |= fc->s_iflags;
	strlcpy(s->s_id, s->s_type->name, sizeof(s->s_id));
	list_add_tail(&s->s_list, &super_blocks);
	hlist_add_head(&s->s_instances, &s->s_type->fs_supers);
	spin_unlock(&sb_lock);
	get_filesystem(s->s_type);
	register_shrinker_prepared(&s->s_shrink);
	return s;
}