Re: [RFC PATCH 3/5] mm, memory_hotplug: allocate memmap from the added memory range for sparse-vmemmap

[Michal Hocko <mhocko@kernel.org>]

On Wed 26-07-17 10:33:31, Michal Hocko wrote:
[...]
> @@ -312,7 +324,7 @@ int __ref __add_pages(int nid, unsigned long phys_start_pfn,
>  	}
>  
>  	for (i = start_sec; i <= end_sec; i++) {
> -		err = __add_section(nid, section_nr_to_pfn(i), want_memblock);
> +		err = __add_section(nid, section_nr_to_pfn(i), want_memblock, altmap);
>  
>  		/*
>  		 * EEXIST is finally dealt with by ioresource collision
[...]
> diff --git a/mm/sparse-vmemmap.c b/mm/sparse-vmemmap.c
> index 483ba270d522..42d6721cfb71 100644
> --- a/mm/sparse-vmemmap.c
> +++ b/mm/sparse-vmemmap.c
> @@ -794,8 +798,20 @@ int __meminit sparse_add_one_section(struct pglist_data *pgdat, unsigned long st
>  		goto out;
>  	}
>  
> +	/*
> +	 * TODO get rid of this somehow - we want to postpone the full
> +	 * initialization until memmap_init_zone.
> +	 */
>  	memset(memmap, 0, sizeof(struct page) * PAGES_PER_SECTION);
>  
> +	/*
> +	 * now that we have a valid vmemmap mapping we can use
> +	 * pfn_to_page and flush struct pages which back the
> +	 * memmap
> +	 */
> +	if (altmap && altmap->flush_alloc_pfns)
> +		altmap->flush_alloc_pfns(altmap);
> +
>  	section_mark_present(ms);
>  
>  	ret = sparse_init_one_section(ms, section_nr, memmap, usemap);

I have only now realized that flush_alloc_pfns would go over the same
range of pfns for each section again and again. I haven't noticed that
because my memblock has one section but larger memblocks (2GB on x86
with a lot of memory) would see that issue. So I will fold the following
into the patch.
---
commit 2658f448af25aa2d2ff7fea12e60a8fe78966f9b
Author: Michal Hocko <mhocko@suse.com>
Date:   Fri Jul 28 19:45:25 2017 +0200

    fold me "mm, memory_hotplug: allocate memmap from the added memory range for sparse-vmemmap"
    
    - rewind base pfn of the vmem_altmap when flushing one section
      (mark_vmemmap_pages) because we do not want to flush the same range
      all over again when memblock has more than one section

diff --git a/include/linux/memremap.h b/include/linux/memremap.h
index 7aec9272fe4d..55f82c652d51 100644
--- a/include/linux/memremap.h
+++ b/include/linux/memremap.h
@@ -18,7 +18,7 @@ struct device;
  *  is mapped to the vmemmap - see mark_vmemmap_pages
  */
 struct vmem_altmap {
-	const unsigned long base_pfn;
+	unsigned long base_pfn;
 	const unsigned long reserve;
 	unsigned long free;
 	unsigned long align;
@@ -48,6 +48,9 @@ static inline void mark_vmemmap_pages(struct vmem_altmap *self)
 		struct page *page = pfn_to_page(pfn);
 		__SetPageVmemmap(page);
 	}
+
+	self->alloc = 0;
+	self->base_pfn += nr_pages + self->reserve;
 }
 /**
  * struct dev_pagemap - metadata for ZONE_DEVICE mappings
-- 
Michal Hocko
SUSE Labs