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Date:   Thu, 2 Nov 2017 14:32:35 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     Pavel Tatashin <pasha.tatashin@...cle.com>
Cc:     steven.sistare@...cle.com, daniel.m.jordan@...cle.com,
        akpm@...ux-foundation.org, mgorman@...hsingularity.net,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1 1/1] mm: buddy page accessed before initialized

On Tue 31-10-17 11:50:02, Pavel Tatashin wrote:
[...]
> The problem happens in this path:
> 
> page_alloc_init_late
>   deferred_init_memmap
>     deferred_init_range
>       __def_free
>         deferred_free_range
>           __free_pages_boot_core(page, order)
>             __free_pages()
>               __free_pages_ok()
>                 free_one_page()
>                   __free_one_page(page, pfn, zone, order, migratetype);
> 
> deferred_init_range() initializes one page at a time by calling
> __init_single_page(), once it initializes pageblock_nr_pages pages, it
> calls deferred_free_range() to free the initialized pages to the buddy
> allocator. Eventually, we reach __free_one_page(), where we compute buddy
> page:
> 	buddy_pfn = __find_buddy_pfn(pfn, order);
> 	buddy = page + (buddy_pfn - pfn);
> 
> buddy_pfn is computed as pfn ^ (1 << order), or pfn + pageblock_nr_pages.
> Thefore, buddy page becomes a page one after the range that currently was
> initialized, and we access this page in this function. Also, later when we
> return back to deferred_init_range(), the buddy page is initialized again.
> 
> So, in order to avoid this issue, we must initialize the buddy page prior
> to calling deferred_free_range().

How come we didn't have this problem previously? I am really confused.

> Signed-off-by: Pavel Tatashin <pasha.tatashin@...cle.com>
> ---
>  mm/page_alloc.c | 10 +++++++++-
>  1 file changed, 9 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 97687b38da05..f3ea06db3eed 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -1500,9 +1500,17 @@ static unsigned long deferred_init_range(int nid, int zid, unsigned long pfn,
>  			__init_single_page(page, pfn, zid, nid);
>  			nr_free++;
>  		} else {
> -			nr_pages += __def_free(&nr_free, &free_base_pfn, &page);
>  			page = pfn_to_page(pfn);
>  			__init_single_page(page, pfn, zid, nid);
> +			/*
> +			 * We must free previous range after initializing the
> +			 * first page of the next range. This is because first
> +			 * page may be accessed in __free_one_page(), when buddy
> +			 * page is computed:
> +			 *   buddy_pfn = pfn + pageblock_nr_pages
> +			 */
> +			deferred_free_range(free_base_pfn, nr_free);
> +			nr_pages += nr_free;
>  			free_base_pfn = pfn;
>  			nr_free = 1;
>  			cond_resched();
> -- 
> 2.14.3
> 

-- 
Michal Hocko
SUSE Labs

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