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Subject: Re: [PATCH v2 3/7] mm/sparse.c: introduce a new function
 clear_subsection_map()
To:     Baoquan He <bhe@redhat.com>
Cc:     linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        akpm@linux-foundation.org, richardw.yang@linux.intel.com,
        osalvador@suse.de, dan.j.williams@intel.com, mhocko@suse.com,
        rppt@linux.ibm.com, robin.murphy@arm.com
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 <20200220043316.19668-4-bhe@redhat.com>
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From:   David Hildenbrand <david@redhat.com>
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Organization: Red Hat GmbH
Message-ID: <53bba557-149c-54b5-c8d7-27f7b6a55eb0@redhat.com>
Date:   Tue, 3 Mar 2020 09:33:06 +0100
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Sender: linux-kernel-owner@vger.kernel.org
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On 03.03.20 09:22, Baoquan He wrote:
> On 03/02/20 at 04:43pm, David Hildenbrand wrote:
>> On 01.03.20 06:20, Baoquan He wrote:
>>> On 02/28/20 at 03:36pm, David Hildenbrand wrote:
>>>> On 20.02.20 05:33, Baoquan He wrote:
>>>>> Wrap the codes which clear subsection map of one memory region from
>>>>> section_deactivate() into clear_subsection_map().
>>>>>
>>>>> Signed-off-by: Baoquan He <bhe@redhat.com>
>>>>> ---
>>>>>  mm/sparse.c | 46 ++++++++++++++++++++++++++++++++++++++--------
>>>>>  1 file changed, 38 insertions(+), 8 deletions(-)
>>>>>
>>>>> diff --git a/mm/sparse.c b/mm/sparse.c
>>>>> index 977b47acd38d..df857ee9330c 100644
>>>>> --- a/mm/sparse.c
>>>>> +++ b/mm/sparse.c
>>>>> @@ -726,14 +726,25 @@ static void free_map_bootmem(struct page *mem=
map)
>>>>>  }
>>>>>  #endif /* CONFIG_SPARSEMEM_VMEMMAP */
>>>>> =20
>>>>> -static void section_deactivate(unsigned long pfn, unsigned long nr=
_pages,
>>>>> -		struct vmem_altmap *altmap)
>>>>> +/**
>>>>> + * clear_subsection_map - Clear subsection map of one memory regio=
n
>>>>> + *
>>>>> + * @pfn - start pfn of the memory range
>>>>> + * @nr_pages - number of pfns to add in the region
>>>>> + *
>>>>> + * This is only intended for hotplug, and clear the related subsec=
tion
>>>>> + * map inside one section.
>>>>> + *
>>>>> + * Return:
>>>>> + * * -EINVAL	- Section already deactived.
>>>>> + * * 0		- Subsection map is emptied.
>>>>> + * * 1		- Subsection map is not empty.
>>>>> + */
>>>>
>>>> Less verbose please (in my preference: none and simplify return hand=
ling)
>>>>
>>>>> +static int clear_subsection_map(unsigned long pfn, unsigned long n=
r_pages)
>>>>>  {
>>>>>  	DECLARE_BITMAP(map, SUBSECTIONS_PER_SECTION) =3D { 0 };
>>>>>  	DECLARE_BITMAP(tmp, SUBSECTIONS_PER_SECTION) =3D { 0 };
>>>>>  	struct mem_section *ms =3D __pfn_to_section(pfn);
>>>>> -	bool section_is_early =3D early_section(ms);
>>>>> -	struct page *memmap =3D NULL;
>>>>>  	unsigned long *subsection_map =3D ms->usage
>>>>>  		? &ms->usage->subsection_map[0] : NULL;
>>>>> =20
>>>>> @@ -744,8 +755,28 @@ static void section_deactivate(unsigned long p=
fn, unsigned long nr_pages,
>>>>>  	if (WARN(!subsection_map || !bitmap_equal(tmp, map, SUBSECTIONS_P=
ER_SECTION),
>>>>>  				"section already deactivated (%#lx + %ld)\n",
>>>>>  				pfn, nr_pages))
>>>>> -		return;
>>>>> +		return -EINVAL;
>>>>> +
>>>>> +	bitmap_xor(subsection_map, map, subsection_map, SUBSECTIONS_PER_S=
ECTION);
>>>>> =20
>>>>> +	if (bitmap_empty(subsection_map, SUBSECTIONS_PER_SECTION))
>>>>> +		return 0;
>>>>> +
>>>>
>>>> Can we please just have a
>>>>
>>>> subsection_map_empty() instead and handle that in the caller?
>>>> (you can then always return true in the !VMEMMAP variant)
>>>
>>> I don't follow. Could you be more specific? or pseudo code please?
>>>
>>> The old code has to handle below case in which subsection_map has bee=
n
>>> cleared. And I introduce clear_subsection_map() to encapsulate all
>>> subsection map realted code so that !VMEMMAP won't have to see it any
>>> more.
>>>
>>
>> Something like this on top would be easier to understand IMHO
>>
>>
>> diff --git a/mm/sparse.c b/mm/sparse.c
>> index dc79b00ddaaa..be5c80e9cfee 100644
>> --- a/mm/sparse.c
>> +++ b/mm/sparse.c
>> @@ -726,20 +726,6 @@ static void free_map_bootmem(struct page *memmap)
>>  }
>>  #endif /* CONFIG_SPARSEMEM_VMEMMAP */
>> =20
>> -/**
>> - * clear_subsection_map - Clear subsection map of one memory region
>> - *
>> - * @pfn - start pfn of the memory range
>> - * @nr_pages - number of pfns to add in the region
>> - *
>> - * This is only intended for hotplug, and clear the related subsectio=
n
>> - * map inside one section.
>> - *
>> - * Return:
>> - * * -EINVAL	- Section already deactived.
>> - * * 0		- Subsection map is emptied.
>> - * * 1		- Subsection map is not empty.
>> - */
>>  static int clear_subsection_map(unsigned long pfn, unsigned long nr_p=
ages)
>>  {
>>  	DECLARE_BITMAP(map, SUBSECTIONS_PER_SECTION) =3D { 0 };
>> @@ -758,11 +744,12 @@ static int clear_subsection_map(unsigned long pf=
n, unsigned long nr_pages)
>>  		return -EINVAL;
>> =20
>>  	bitmap_xor(subsection_map, map, subsection_map, SUBSECTIONS_PER_SECT=
ION);
>> +	return 0;
>> +}
>> =20
>> -	if (bitmap_empty(subsection_map, SUBSECTIONS_PER_SECTION))
>> -		return 0;
>> -
>> -	return 1;
>> +static bool is_subsection_map_empty(unsigned long pfn, unsigned long =
nr_pages)
>> +{
>> +	return bitmap_empty(subsection_map, SUBSECTIONS_PER_SECTION);
>>  }
>> =20
>>  static void section_deactivate(unsigned long pfn, unsigned long nr_pa=
ges,
>> @@ -771,11 +758,8 @@ static void section_deactivate(unsigned long pfn,=
 unsigned long nr_pages,
>>  	struct mem_section *ms =3D __pfn_to_section(pfn);
>>  	bool section_is_early =3D early_section(ms);
>>  	struct page *memmap =3D NULL;
>> -	int rc;
>> -
>> =20
>> -	rc =3D clear_subsection_map(pfn, nr_pages);
>> -	if (IS_ERR_VALUE((unsigned long)rc))
>> +	if (unlikely(clear_subsection_map(pfn, nr_pages)))
>>  		return;
>>  	/*
>>  	 * There are 3 cases to handle across two configurations
>> @@ -794,7 +778,7 @@ static void section_deactivate(unsigned long pfn, =
unsigned long nr_pages,
>>  	 *
>>  	 * For 2/ and 3/ the SPARSEMEM_VMEMMAP=3D{y,n} cases are unified
>>  	 */
>> -	if (!rc) {
>> +	if (is_subsection_map_empty(pfn, nr_pages)) {
>>  		unsigned long section_nr =3D pfn_to_section_nr(pfn);
>=20
> Tried this way, it's not good in this patch. Since ms->usage might be
> freed in this place.
>=20
>                 if (!PageReserved(virt_to_page(ms->usage))) {
>                         kfree(ms->usage);
>                         ms->usage =3D NULL;
>                 }

So your patch #1 is already broken. Just cache the result in patch #1.

bool empty;

...
empty =3D bitmap_empty(subsection_map, SUBSECTIONS_PER_SECTION);
...
if (empty) {
	...
}

--=20
Thanks,

David / dhildenb