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Subject: Re: [PATCH 1/7] mm/sparse.c: Introduce new function
 fill_subsection_map()
To:     Baoquan He <bhe@redhat.com>
Cc:     linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        akpm@linux-foundation.org, dan.j.williams@intel.com,
        richardw.yang@linux.intel.com
References: <20200209104826.3385-1-bhe@redhat.com>
 <20200209104826.3385-2-bhe@redhat.com>
 <0463a54b-12cb-667e-7c86-66cd707cec84@redhat.com>
 <20200211124648.GF8965@MiWiFi-R3L-srv>
From:   David Hildenbrand <david@redhat.com>
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Organization: Red Hat GmbH
Message-ID: <659d16c9-79f7-a1d6-ca05-d6164c9c11a6@redhat.com>
Date:   Tue, 11 Feb 2020 15:44:14 +0100
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On 11.02.20 13:46, Baoquan He wrote:
> On 02/10/20 at 10:49am, David Hildenbrand wrote:
>> On 09.02.20 11:48, Baoquan He wrote:
>>> Wrap the codes filling subsection map in section_activate() into
>>> fill_subsection_map(), this makes section_activate() cleaner and
>>> easier to follow.
>>>
>>> Signed-off-by: Baoquan He <bhe@redhat.com>
>>> ---
>>>  mm/sparse.c | 45 ++++++++++++++++++++++++++++++++++-----------
>>>  1 file changed, 34 insertions(+), 11 deletions(-)
>>>
>>> diff --git a/mm/sparse.c b/mm/sparse.c
>>> index c184b69460b7..9ad741ccbeb6 100644
>>> --- a/mm/sparse.c
>>> +++ b/mm/sparse.c
>>> @@ -788,24 +788,28 @@ static void section_deactivate(unsigned long pf=
n, unsigned long nr_pages,
>>>  		depopulate_section_memmap(pfn, nr_pages, altmap);
>>>  }
>>> =20
>>> -static struct page * __meminit section_activate(int nid, unsigned lo=
ng pfn,
>>> -		unsigned long nr_pages, struct vmem_altmap *altmap)
>>> +/**
>>> + * fill_subsection_map - fill subsection map of a memory region
>>> + * @pfn - start pfn of the memory range
>>> + * @nr_pages - number of pfns to add in the region
>>> + *
>>> + * This clears the related subsection map inside one section, and on=
ly
>>> + * intended for hotplug.
>>> + *
>>> + * Return:
>>> + * * 0		- On success.
>>> + * * -EINVAL	- Invalid memory region.
>>> + * * -EEXIST	- Subsection map has been set.
>>> + */
>>> +static int fill_subsection_map(unsigned long pfn, unsigned long nr_p=
ages)
>>>  {
>>> -	DECLARE_BITMAP(map, SUBSECTIONS_PER_SECTION) =3D { 0 };
>>>  	struct mem_section *ms =3D __pfn_to_section(pfn);
>>> -	struct mem_section_usage *usage =3D NULL;
>>> +	DECLARE_BITMAP(map, SUBSECTIONS_PER_SECTION) =3D { 0 };
>>>  	unsigned long *subsection_map;
>>> -	struct page *memmap;
>>>  	int rc =3D 0;
>>> =20
>>>  	subsection_mask_set(map, pfn, nr_pages);
>>> =20
>>> -	if (!ms->usage) {
>>> -		usage =3D kzalloc(mem_section_usage_size(), GFP_KERNEL);
>>> -		if (!usage)
>>> -			return ERR_PTR(-ENOMEM);
>>> -		ms->usage =3D usage;
>>> -	}
>>>  	subsection_map =3D &ms->usage->subsection_map[0];
>>> =20
>>>  	if (bitmap_empty(map, SUBSECTIONS_PER_SECTION))
>>> @@ -816,6 +820,25 @@ static struct page * __meminit section_activate(=
int nid, unsigned long pfn,
>>>  		bitmap_or(subsection_map, map, subsection_map,
>>>  				SUBSECTIONS_PER_SECTION);
>>> =20
>>> +	return rc;
>>> +}
>>> +
>>> +static struct page * __meminit section_activate(int nid, unsigned lo=
ng pfn,
>>> +		unsigned long nr_pages, struct vmem_altmap *altmap)
>>> +{
>>> +	struct mem_section *ms =3D __pfn_to_section(pfn);
>>> +	struct mem_section_usage *usage =3D NULL;
>>> +	struct page *memmap;
>>> +	int rc =3D 0;
>>> +
>>> +	if (!ms->usage) {
>>> +		usage =3D kzalloc(mem_section_usage_size(), GFP_KERNEL);
>>> +		if (!usage)
>>> +			return ERR_PTR(-ENOMEM);
>>> +		ms->usage =3D usage;
>>> +	}
>>> +
>>> +	rc =3D fill_subsection_map(pfn, nr_pages);
>>>  	if (rc) {
>>>  		if (usage)
>>>  			ms->usage =3D NULL;
>>>
>>
>> What about having two variants of
>> section_activate()/section_deactivate() instead? Then we don't have an=
y
>> subsection related stuff in !subsection code.
>=20
> Thanks for looking into this, David.
>=20
> Having two variants of section_activate()/section_deactivate() is also
> good. Just not like memmap handling which is very different between cla=
ssic
> sparse and vmemmap, makes having two variants very attractive, the code
> and logic in section_activate()/section_deactivate() is not too much,
> and both of them basically can share the most of code, these make the
> variants way not so necessary. I personally prefer the current way, wha=
t
> do you think?

I was looking at

if (nr_pages < PAGES_PER_SECTION && early_section(ms))
	return pfn_to_page(pfn);

and thought that it is also specific to sub-section handling. I wonder
if we can simply move that into the VMEMMAP variant of
populate_section_memmap()?

But apart from that I agree that the end result with the current
approach is also nice.

Can you reshuffle the patches, moving the fixes to the very front so we
can backport more easily?

--=20
Thanks,

David / dhildenb