Re: [PATCH v1 2/2] mm: mTHP stats for pagecache folio allocations

[Baolin Wang]

On 2024/7/14 17:05, Ryan Roberts wrote:
> On 13/07/2024 13:54, Baolin Wang wrote:
> > On 2024/7/13 19:00, Ryan Roberts wrote:
> > > [...]
> > >
> > > > > +static int thpsize_create(int order, struct kobject *parent)
> > > > >     {
> > > > >         unsigned long size = (PAGE_SIZE << order) / SZ_1K;
> > > > > +    struct thpsize_child *stats;
> > > > >         struct thpsize *thpsize;
> > > > >         int ret;
> > > > >     +    /*
> > > > > +     * Each child object (currently only "stats" directory) holds a
> > > > > +     * reference to the top-level thpsize object, so we can drop our ref to
> > > > > +     * the top-level once stats is setup. Then we just need to drop a
> > > > > +     * reference on any children to clean everything up. We can't just use
> > > > > +     * the attr group name for the stats subdirectory because there may be
> > > > > +     * multiple attribute groups to populate inside stats and overlaying
> > > > > +     * using the name property isn't supported in that way; each attr group
> > > > > +     * name, if provided, must be unique in the parent directory.
> > > > > +     */
> > > > > +
> > > > >         thpsize = kzalloc(sizeof(*thpsize), GFP_KERNEL);
> > > > > -    if (!thpsize)
> > > > > -        return ERR_PTR(-ENOMEM);
> > > > > +    if (!thpsize) {
> > > > > +        ret = -ENOMEM;
> > > > > +        goto err;
> > > > > +    }
> > > > > +    thpsize->order = order;
> > > > >           ret = kobject_init_and_add(&thpsize->kobj, &thpsize_ktype, parent,
> > > > >                        "hugepages-%lukB", size);
> > > > >         if (ret) {
> > > > >             kfree(thpsize);
> > > > > -        return ERR_PTR(ret);
> > > > > +        goto err;
> > > > >         }
> > > > >     -    ret = sysfs_create_group(&thpsize->kobj, &thpsize_attr_group);
> > > > > -    if (ret) {
> > > > > +    stats = kzalloc(sizeof(*stats), GFP_KERNEL);
> > > > > +    if (!stats) {
> > > > >             kobject_put(&thpsize->kobj);
> > > > > -        return ERR_PTR(ret);
> > > > > +        ret = -ENOMEM;
> > > > > +        goto err;
> > > > >         }
> > > > >     -    ret = sysfs_create_group(&thpsize->kobj, &stats_attr_group);
> > > > > +    ret = kobject_init_and_add(&stats->kobj, &thpsize_child_ktype,
> > > > > +                   &thpsize->kobj, "stats");
> > > > > +    kobject_put(&thpsize->kobj);
> > > > >         if (ret) {
> > > > > -        kobject_put(&thpsize->kobj);
> > > > > -        return ERR_PTR(ret);
> > > > > +        kfree(stats);
> > > > > +        goto err;
> > > > >         }
> > > > >     -    thpsize->order = order;
> > > > > -    return thpsize;
> > > > > +    if (BIT(order) & THP_ORDERS_ALL_ANON) {
> > > > > +        ret = sysfs_create_group(&thpsize->kobj, &thpsize_attr_group);
> > > > > +        if (ret)
> > > > > +            goto err_put;
> > > > > +
> > > > > +        ret = sysfs_create_group(&stats->kobj, &stats_attr_group);
> > > > > +        if (ret)
> > > > > +            goto err_put;
> > > > > +    }
> > > > > +
> > > > > +    if (BIT(order) & PAGECACHE_LARGE_ORDERS) {
> > > > > +        ret = sysfs_create_group(&stats->kobj, &file_stats_attr_group);
> > > > > +        if (ret)
> > > > > +            goto err_put;
> > > > > +    }
> > > > > +
> > > > > +    list_add(&stats->node, &thpsize_child_list);
> > > > > +    return 0;
> > > > > +err_put:
> > > >
> > > > IIUC, I think you should call 'sysfs_remove_group' to remove the group before
> > > > putting the kobject.
> > >
> > > Are you sure about that? As I understood it, sysfs_create_group() was
> > > conceptually modifying the state of the kobj, so when the kobj gets destroyed,
> > > all its state is tidied up. __kobject_del() (called on the last kobject_put())
> > > calls sysfs_remove_groups() and tidies up the sysfs state as far as I can see?
> >
> > IIUC, __kobject_del() only removes the ktype defaut groups by
> > 'sysfs_remove_groups(kobj, ktype->default_groups)', but your created groups are
> > not added into the ktype->default_groups. That means you should mannuly remove
> > them, or am I miss something?
>
> That was also putting doubt in my mind. But the sample at
> samples/kobject/kobject-example.c does not call sysfs_remove_group(). It just
> calls sysfs_create_group() in example_init() and calls kobject_put() in
> example_exit(). So I think that's the correct pattern.
>
> Looking at the code more closely, sysfs_create_group() just creates files for
> each of the attributes in the group. __kobject_del() calls sysfs_remove_dir(),
> who's comment states "we remove any files in the directory before we remove the
> directory" so I'm pretty sure sysfs_remove_group() is not required.

Thanks for the explanation, and I think you are right after checking the 
code again. Sorry for the noise.