linux-kernel - Re: [PATCH 1/7] mm/sparse.c: Introduce new function fill_subsection

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Date:   Tue, 11 Feb 2020 15:44:14 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Baoquan He <bhe@...hat.com>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        akpm@...ux-foundation.org, dan.j.williams@...el.com,
        richardw.yang@...ux.intel.com
Subject: Re: [PATCH 1/7] mm/sparse.c: Introduce new function
 fill_subsection_map()

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@...hat.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 pfn, unsigned long nr_pages,
>>>  		depopulate_section_memmap(pfn, nr_pages, altmap);
>>>  }
>>>  
>>> -static struct page * __meminit section_activate(int nid, unsigned long 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 only
>>> + * 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_pages)
>>>  {
>>> -	DECLARE_BITMAP(map, SUBSECTIONS_PER_SECTION) = { 0 };
>>>  	struct mem_section *ms = __pfn_to_section(pfn);
>>> -	struct mem_section_usage *usage = NULL;
>>> +	DECLARE_BITMAP(map, SUBSECTIONS_PER_SECTION) = { 0 };
>>>  	unsigned long *subsection_map;
>>> -	struct page *memmap;
>>>  	int rc = 0;
>>>  
>>>  	subsection_mask_set(map, pfn, nr_pages);
>>>  
>>> -	if (!ms->usage) {
>>> -		usage = kzalloc(mem_section_usage_size(), GFP_KERNEL);
>>> -		if (!usage)
>>> -			return ERR_PTR(-ENOMEM);
>>> -		ms->usage = usage;
>>> -	}
>>>  	subsection_map = &ms->usage->subsection_map[0];
>>>  
>>>  	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);
>>>  
>>> +	return rc;
>>> +}
>>> +
>>> +static struct page * __meminit section_activate(int nid, unsigned long pfn,
>>> +		unsigned long nr_pages, struct vmem_altmap *altmap)
>>> +{
>>> +	struct mem_section *ms = __pfn_to_section(pfn);
>>> +	struct mem_section_usage *usage = NULL;
>>> +	struct page *memmap;
>>> +	int rc = 0;
>>> +
>>> +	if (!ms->usage) {
>>> +		usage = kzalloc(mem_section_usage_size(), GFP_KERNEL);
>>> +		if (!usage)
>>> +			return ERR_PTR(-ENOMEM);
>>> +		ms->usage = usage;
>>> +	}
>>> +
>>> +	rc = fill_subsection_map(pfn, nr_pages);
>>>  	if (rc) {
>>>  		if (usage)
>>>  			ms->usage = NULL;
>>>
>>
>> What about having two variants of
>> section_activate()/section_deactivate() instead? Then we don't have any
>> subsection related stuff in !subsection code.
> 
> Thanks for looking into this, David.
> 
> Having two variants of section_activate()/section_deactivate() is also
> good. Just not like memmap handling which is very different between classic
> 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, what
> 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?

-- 
Thanks,

David / dhildenb