linux-kernel - Re: [PATCH v1] mm: inititalize struct pages when adding a section

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Date:   Fri, 27 Jul 2018 13:25:45 -0400
From:   Pavel Tatashin <pasha.tatashin@...cle.com>
To:     david@...hat.com
Cc:     Linux Memory Management List <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>, gregkh@...uxfoundation.org,
        mingo@...nel.org, Andrew Morton <akpm@...ux-foundation.org>,
        dan.j.williams@...el.com, Michal Hocko <mhocko@...e.com>,
        jack@...e.cz, mawilcox@...rosoft.com, jglisse@...hat.com,
        Souptick Joarder <jrdr.linux@...il.com>,
        kirill.shutemov@...ux.intel.com, Vlastimil Babka <vbabka@...e.cz>,
        osalvador@...hadventures.net, yasu.isimatu@...il.com,
        malat@...ian.org, Mel Gorman <mgorman@...e.de>,
        iamjoonsoo.kim@....com
Subject: Re: [PATCH v1] mm: inititalize struct pages when adding a section

Hi David,

On Fri, Jul 27, 2018 at 12:55 PM David Hildenbrand <david@...hat.com> wrote:
>
> Right now, struct pages are inititalized when memory is onlined, not
> when it is added (since commit d0dc12e86b31 ("mm/memory_hotplug: optimize
> memory hotplug")).
>
> remove_memory() will call arch_remove_memory(). Here, we usually access
> the struct page to get the zone of the pages.
>
> So effectively, we access stale struct pages in case we remove memory that
> was never onlined.

Yeah, this is a bug, thank you for catching it.

> So let's simply inititalize them earlier, when the
> memory is added. We only have to take care of updating the zone once we
> know it. We can use a dummy zone for that purpose.
>
> So effectively, all pages will already be initialized and set to
> reserved after memory was added but before it was onlined (and even the
> memblock is added). We only inititalize pages once, to not degrade
> performance.

Yes, but we still add one more npages loop, so there will be some
performance degradation, but not severe.

There are many conflicts with linux-next, please sync before sending
out next patch.

> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -1162,7 +1162,15 @@ static inline void set_page_address(struct page *page, void *address)
>  {
>         page->virtual = address;
>  }
> +static void set_page_virtual(struct page *page, and enum zone_type zone)
> +{
> +       /* The shift won't overflow because ZONE_NORMAL is below 4G. */
> +       if (!is_highmem_idx(zone))
> +               set_page_address(page, __va(pfn << PAGE_SHIFT));
> +}
>  #define page_address_init()  do { } while(0)
> +#else
> +#define set_page_virtual(page, zone)  do { } while(0)
>  #endif

Please use inline functions for both if WANT_PAGE_VIRTUAL case and else case.

>  #if defined(HASHED_PAGE_VIRTUAL)
> @@ -2116,6 +2124,8 @@ extern unsigned long find_min_pfn_with_active_regions(void);
>  extern void free_bootmem_with_active_regions(int nid,
>                                                 unsigned long max_low_pfn);
>  extern void sparse_memory_present_with_active_regions(int nid);
> +extern void __meminit init_single_page(struct page *page, unsigned long pfn,
> +                                      unsigned long zone, int nid);

I do not like making init_single_page() public. There is less chance
it is going to be inlined. I think a better way is to have a new
variant of memmap_init_zone that will handle hotplug case.

>
>  #endif /* CONFIG_HAVE_MEMBLOCK_NODE_MAP */
>
> diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
> index 7deb49f69e27..3f28ca3c3a33 100644
> --- a/mm/memory_hotplug.c
> +++ b/mm/memory_hotplug.c
> @@ -250,6 +250,7 @@ static int __meminit __add_section(int nid, unsigned long phys_start_pfn,
>                 struct vmem_altmap *altmap, bool want_memblock)
>  {
>         int ret;
> +       int i;
>
>         if (pfn_valid(phys_start_pfn))
>                 return -EEXIST;
> @@ -258,6 +259,23 @@ static int __meminit __add_section(int nid, unsigned long phys_start_pfn,
>         if (ret < 0)
>                 return ret;
>
> +       /*
> +        * Initialize all pages in the section before fully exposing them to the
> +        * system so nobody will stumble over a half inititalized state.
> +        */
> +       for (i = 0; i < PAGES_PER_SECTION; i++) {
> +               unsigned long pfn = phys_start_pfn + i;
> +               struct page *page;
> +
> +               if (!pfn_valid(pfn))
> +                       continue;
> +               page = pfn_to_page(pfn);
> +
> +               /* dummy zone, the actual one will be set when onlining pages */
> +               init_single_page(page, pfn, ZONE_NORMAL, nid);
> +               SetPageReserved(page);
> +       }

Please move all of the above into a new memmap_init_hotplug() that
should be located in page_alloc.c


> @@ -5519,9 +5515,12 @@ void __meminit memmap_init_zone(unsigned long size, int nid, unsigned long zone,
>
>  not_early:
>                 page = pfn_to_page(pfn);
> -               __init_single_page(page, pfn, zone, nid);
> -               if (context == MEMMAP_HOTPLUG)
> -                       SetPageReserved(page);
> +               if (context == MEMMAP_HOTPLUG) {
> +                       /* everything but the zone was inititalized */
> +                       set_page_zone(page, zone);
> +                       set_page_virtual(page, zone);
> +               } else
> +                       init_single_page(page, pfn, zone, nid);
>

Please add a new function:
memmap_init_zone_hotplug() that will handle only the zone and virtual
fields for onlined hotplug memory.

Please remove: "enum memmap_context context" from everywhere.

Thank you,
Pavel