Re: [PATCH v2] Revert "mm/page_alloc: fix memmap_init_zone pageblock alignment"

[Jan Glauber]

On Wed, Mar 14, 2018 at 07:29:37PM +0000, Ard Biesheuvel wrote:
> This reverts commit 864b75f9d6b0100bb24fdd9a20d156e7cda9b5ae.

FWIW, the revert fixes the boot hang I'm seeing on ThunderX1.

--Jan

> Commit 864b75f9d6b0 ("mm/page_alloc: fix memmap_init_zone pageblock
> alignment") modified the logic in memmap_init_zone() to initialize
> struct pages associated with invalid PFNs, to appease a VM_BUG_ON()
> in move_freepages(), which is redundant by its own admission, and
> dereferences struct page fields to obtain the zone without checking
> whether the struct pages in question are valid to begin with.
> 
> Commit 864b75f9d6b0 only makes it worse, since the rounding it does
> may cause pfn assume the same value it had in a prior iteration of
> the loop, resulting in an infinite loop and a hang very early in the
> boot. Also, since it doesn't perform the same rounding on start_pfn
> itself but only on intermediate values following an invalid PFN, we
> may still hit the same VM_BUG_ON() as before.
> 
> So instead, let's fix this at the core, and ensure that the BUG
> check doesn't dereference struct page fields of invalid pages.
> 
> Fixes: 864b75f9d6b0 ("mm/page_alloc: fix memmap_init_zone pageblock alignment")
> Cc: Daniel Vacek <neelx@xxxxxxxxxx>
> Cc: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
> Cc: Michal Hocko <mhocko@xxxxxxxx>
> Cc: Paul Burton <paul.burton@xxxxxxxxxx>
> Cc: Pavel Tatashin <pasha.tatashin@xxxxxxxxxx>
> Cc: Vlastimil Babka <vbabka@xxxxxxx>
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
> Signed-off-by: Ard Biesheuvel <ard.biesheuvel@xxxxxxxxxx>
> ---
>  mm/page_alloc.c | 13 +++++--------
>  1 file changed, 5 insertions(+), 8 deletions(-)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 3d974cb2a1a1..635d7dd29d7f 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -1910,7 +1910,9 @@ static int move_freepages(struct zone *zone,
>  	 * Remove at a later date when no bug reports exist related to
>  	 * grouping pages by mobility
>  	 */
> -	VM_BUG_ON(page_zone(start_page) != page_zone(end_page));
> +	VM_BUG_ON(pfn_valid(page_to_pfn(start_page)) &&
> +	          pfn_valid(page_to_pfn(end_page)) &&
> +	          page_zone(start_page) != page_zone(end_page));
>  #endif
>  
>  	if (num_movable)
> @@ -5359,14 +5361,9 @@ 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. Note that it needs
> -			 * to be pageblock aligned even when the region itself
> -			 * is not. move_freepages_block() can shift ahead of
> -			 * the valid region but still depends on correct page
> -			 * metadata.
> +			 * on our next iteration of the loop.
>  			 */
> -			pfn = (memblock_next_valid_pfn(pfn, end_pfn) &
> -					~(pageblock_nr_pages-1)) - 1;
> +			pfn = memblock_next_valid_pfn(pfn, end_pfn) - 1;
>  #endif
>  			continue;
>  		}
> -- 
> 2.15.1
> 
> 
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