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

[Shanker Donthineni]

Hi Ard,

On 03/14/2018 05:25 PM, Jan Glauber wrote:
> 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
> 

Thanks for this patch, it fixes the boot hang on QDF2400 platform.


>> 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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> 
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> 

-- 
Shanker Donthineni
Qualcomm Datacenter Technologies, Inc. as an affiliate of Qualcomm Technologies, Inc.
Qualcomm Technologies, Inc. is a member of the Code Aurora Forum, a Linux Foundation Collaborative Project.