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

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Message-ID: <b8d1dc3c-ee05-450e-961e-b13dded06a78@linux.alibaba.com>
Date: Sat, 13 Jul 2024 20:54:06 +0800
From: Baolin Wang <baolin.wang@...ux.alibaba.com>
To: Ryan Roberts <ryan.roberts@....com>,
 Andrew Morton <akpm@...ux-foundation.org>, Hugh Dickins <hughd@...gle.com>,
 Jonathan Corbet <corbet@....net>,
 "Matthew Wilcox (Oracle)" <willy@...radead.org>,
 David Hildenbrand <david@...hat.com>, Barry Song <baohua@...nel.org>,
 Lance Yang <ioworker0@...il.com>
Cc: linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH v1 2/2] mm: mTHP stats for pagecache folio allocations



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?