linux-kernel - Re: [PATCH] mm/page_alloc: fix memmap_init

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Date:   Thu, 1 Mar 2018 14:10:33 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     Daniel Vacek <neelx@...hat.com>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Pavel Tatashin <pasha.tatashin@...cle.com>,
        Paul Burton <paul.burton@...tec.com>, stable@...r.kernel.org
Subject: Re: [PATCH] mm/page_alloc: fix memmap_init_zone pageblock alignment

On Thu 01-03-18 13:47:45, Daniel Vacek wrote:
> In move_freepages() a BUG_ON() can be triggered on uninitialized page structures
> due to pageblock alignment. Aligning the skipped pfns in memmap_init_zone() the
> same way as in move_freepages_block() simply fixes those crashes.

This changelog doesn't describe how the fix works. Why doesn't
memblock_next_valid_pfn return the first valid pfn as one would expect?

It would be also good put the panic info in the changelog.

> Fixes: b92df1de5d28 ("[mm] page_alloc: skip over regions of invalid pfns where possible")
> Signed-off-by: Daniel Vacek <neelx@...hat.com>
> Cc: stable@...r.kernel.org
> ---
>  mm/page_alloc.c | 9 +++++++--
>  1 file changed, 7 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index cb416723538f..9edee36e6a74 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -5359,9 +5359,14 @@ void __meminit memmap_init_zone(unsigned long size, int nid, unsigned long zone,
>  			/*
>  			 * Skip to the pfn preceding the next valid one (or
>  			 * end_pfn), such that we hit a valid pfn (or end_pfn)
> -			 * on our next iteration of the loop.
> +			 * on our next iteration of the loop. Note that it needs
> +			 * to be pageblock aligned even when the region itself
> +			 * is not as move_freepages_block() can shift ahead of
> +			 * the valid region but still depends on correct page
> +			 * metadata.
>  			 */
> -			pfn = memblock_next_valid_pfn(pfn, end_pfn) - 1;
> +			pfn = (memblock_next_valid_pfn(pfn, end_pfn) &
> +						~(pageblock_nr_pages-1)) - 1;
>  #endif
>  			continue;
>  		}
> -- 
> 2.16.2
> 

-- 
Michal Hocko
SUSE Labs