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    SubjectRe: [PATCH v2] vfs: Pre-allocate superblock in sget_fc() if !test
    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@redhat.com>
    > ---
    > 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;
    }

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