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

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Message-ID: <2109a518-8c5e-41c8-8bd4-9c3499c29fd0@arm.com>
Date: Mon, 22 Jul 2024 08:36:51 +0100
From: Ryan Roberts <ryan.roberts@....com>
To: Baolin Wang <baolin.wang@...ux.alibaba.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 22/07/2024 04:52, Baolin Wang wrote:
> 
> 
> 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.

No problem, thanks for raising anyway; TBH, I wasn't completely sure when I
wrote it initially. So good to have clear resolution.