Date:   Thu, 5 Jul 2018 06:39:25 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Pavel Tatashin <pasha.tatashin@...cle.com>
Cc:     Steven Sistare <steven.sistare@...cle.com>,
        Daniel Jordan <daniel.m.jordan@...cle.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        kirill.shutemov@...ux.intel.com, Michal Hocko <mhocko@...e.com>,
        Linux Memory Management List <linux-mm@...ck.org>,
        dan.j.williams@...el.com, jack@...e.cz, jglisse@...hat.com,
        Souptick Joarder <jrdr.linux@...il.com>, bhe@...hat.com,
        gregkh@...uxfoundation.org, Vlastimil Babka <vbabka@...e.cz>,
        Wei Yang <richard.weiyang@...il.com>, rientjes@...gle.com,
        mingo@...nel.org, osalvador@...hadventures.net
Subject: Re: [PATCH v3 1/2] mm/sparse: add sparse_init_nid()

On 07/02/2018 01:29 PM, Pavel Tatashin wrote:
> On Mon, Jul 2, 2018 at 4:00 PM Dave Hansen <dave.hansen@...el.com> wrote:
>>> +     unsigned long size = sizeof(struct page) * PAGES_PER_SECTION;
>>> +     unsigned long pnum, map_index = 0;
>>> +     void *vmemmap_buf_start;
>>> +
>>> +     size = ALIGN(size, PMD_SIZE) * map_count;
>>> +     vmemmap_buf_start = __earlyonly_bootmem_alloc(nid, size,
>>> +                                                   PMD_SIZE,
>>> +                                                   __pa(MAX_DMA_ADDRESS));
>>
>> Let's not repeat the mistakes of the previous version of the code.
>> Please explain why we are aligning this.  Also,
>> __earlyonly_bootmem_alloc()->memblock_virt_alloc_try_nid_raw() claims to
>> be aligning the size.  Do we also need to do it here?
>>
>> Yes, I know the old code did this, but this is the cost of doing a
>> rewrite. :)
> 
> Actually, I was thinking about this particular case when I was
> rewriting this code. Here we align size before multiplying by
> map_count aligns after memblock_virt_alloc_try_nid_raw(). So, we must
> have both as they are different.

That's a good point that they do different things.

But, which behavior of the two different things is the one we _want_?

>>> +     if (vmemmap_buf_start) {
>>> +             vmemmap_buf = vmemmap_buf_start;
>>> +             vmemmap_buf_end = vmemmap_buf_start + size;
>>> +     }
>>
>> It would be nice to call out that these are globals that other code
>> picks up.
> 
> I do not like these globals, they should have specific functions that
> access them only, something:
> static struct {
>   buffer;
>   buffer_end;
> } vmemmap_buffer;
> vmemmap_buffer_init() allocate buffer
> vmemmap_buffer_alloc()  return NULL if buffer is empty
> vmemmap_buffer_fini()
> 
> Call vmemmap_buffer_init()  and vmemmap_buffer_fini()  from
> sparse_populate_node() and
> vmemmap_buffer_alloc() from vmemmap_alloc_block_buf().
> 
> But, it should be a separate patch. If you would like I can add it to
> this series, or submit separately.

Seems like a nice cleanup, but I don't think it needs to be done here.

>>> + * Return map for pnum section. sparse_populate_node() has populated memory map
>>> + * in this node, we simply do pnum to struct page conversion.
>>> + */
>>> +struct page * __init sparse_populate_node_section(struct page *map_base,
>>> +                                               unsigned long map_index,
>>> +                                               unsigned long pnum,
>>> +                                               int nid)
>>> +{
>>> +     return pfn_to_page(section_nr_to_pfn(pnum));
>>> +}
>>
>> What is up with all of the unused arguments to this function?
> 
> Because the same function is called from non-vmemmap sparse code.

That's probably good to call out in the patch description if not there
already.

>>> diff --git a/mm/sparse.c b/mm/sparse.c
>>> index d18e2697a781..c18d92b8ab9b 100644
>>> --- a/mm/sparse.c
>>> +++ b/mm/sparse.c
>>> @@ -456,6 +456,43 @@ void __init sparse_mem_maps_populate_node(struct page **map_map,
>>>                      __func__);
>>>       }
>>>  }
>>> +
>>> +static unsigned long section_map_size(void)
>>> +{
>>> +     return PAGE_ALIGN(sizeof(struct page) * PAGES_PER_SECTION);
>>> +}
>>
>> Seems like if we have this, we should use it wherever possible, like
>> sparse_populate_node().
> 
> It is used in sparse_populate_node():
> 
> 401 struct page * __init sparse_populate_node(unsigned long pnum_begin,
> 406         return memblock_virt_alloc_try_nid_raw(section_map_size()
> * map_count,
> 407                                                PAGE_SIZE,
> __pa(MAX_DMA_ADDRESS),
> 408
> BOOTMEM_ALLOC_ACCESSIBLE, nid);

I missed the PAGE_ALIGN() until now.  That really needs a comment
calling out how it's not really the map size but the *allocation* size
of a single section's map.

It probably also needs a name like section_memmap_allocation_size() or
something to differentiate it from the *used* size.

>>> +/*
>>> + * Try to allocate all struct pages for this node, if this fails, we will
>>> + * be allocating one section at a time in sparse_populate_node_section().
>>> + */
>>> +struct page * __init sparse_populate_node(unsigned long pnum_begin,
>>> +                                       unsigned long pnum_end,
>>> +                                       unsigned long map_count,
>>> +                                       int nid)
>>> +{
>>> +     return memblock_virt_alloc_try_nid_raw(section_map_size() * map_count,
>>> +                                            PAGE_SIZE, __pa(MAX_DMA_ADDRESS),
>>> +                                            BOOTMEM_ALLOC_ACCESSIBLE, nid);
>>> +}
>>> +
>>> +/*
>>> + * Return map for pnum section. map_base is not NULL if we could allocate map
>>> + * for this node together. Otherwise we allocate one section at a time.
>>> + * map_index is the index of pnum in this node counting only present sections.
>>> + */
>>> +struct page * __init sparse_populate_node_section(struct page *map_base,
>>> +                                               unsigned long map_index,
>>> +                                               unsigned long pnum,
>>> +                                               int nid)
>>> +{
>>> +     if (map_base) {
>>> +             unsigned long offset = section_map_size() * map_index;
>>> +
>>> +             return (struct page *)((char *)map_base + offset);
>>> +     }
>>> +     return sparse_mem_map_populate(pnum, nid, NULL);
>>
>> Oh, you have a vmemmap and non-vmemmap version.
>>
>> BTW, can't the whole map base calculation just be replaced with:
>>
>>         return &map_base[PAGES_PER_SECTION * map_index];
> 
> Unfortunately no.  Because map_base might be allocated in chunks
> larger than PAGES_PER_SECTION * sizeof(struct page). See: PAGE_ALIGN()
> in section_map_size

Good point.

Oh, well, can you at least get rid of the superfluous "(char *)" cast?
That should make the whole thing a bit less onerous.

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