Re: [PATCH RFC] mm: fix regression with deferred struct page init

[Juergen Gross <jgross@suse.com>]

On 20.06.19 17:17, Alexander Duyck wrote:
> On Thu, 2019-06-20 at 11:40 +0200, Juergen Gross wrote:
>> Commit 0e56acae4b4dd4a9 ("mm: initialize MAX_ORDER_NR_PAGES at a time
>> instead of doing larger sections") is causing a regression on some
>> systems when the kernel is booted as Xen dom0.
>>
>> The system will just hang in early boot.
>>
>> Reason is an endless loop in get_page_from_freelist() in case the first
>> zone looked at has no free memory. deferred_grow_zone() is always
>> returning true due to the following code snipplet:
>>
>>    /* If the zone is empty somebody else may have cleared out the zone */
>>    if (!deferred_init_mem_pfn_range_in_zone(&i, zone, &spfn, &epfn,
>>                                             first_deferred_pfn)) {
>>            pgdat->first_deferred_pfn = ULONG_MAX;
>>            pgdat_resize_unlock(pgdat, &flags);
>>            return true;
>>    }
>>
>> This in turn results in the loop as get_page_from_freelist() is
>> assuming forward progress can be made by doing some more struct page
>> initialization.
>>
>> Fixes: 0e56acae4b4dd4a9 ("mm: initialize MAX_ORDER_NR_PAGES at a time instead of doing larger sections")
>> ---
>> This patch makes my system boot again as Xen dom0, but I'm not really
>> sure it is the correct way to do it, hence the RFC.
>> Signed-off-by: Juergen Gross <jgross@suse.com>
>> ---
>>   mm/page_alloc.c | 2 +-
>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
>> index d66bc8abe0af..6ee754b5cd92 100644
>> --- a/mm/page_alloc.c
>> +++ b/mm/page_alloc.c
>> @@ -1826,7 +1826,7 @@ deferred_grow_zone(struct zone *zone, unsigned int order)
>>   						 first_deferred_pfn)) {
>>   		pgdat->first_deferred_pfn = ULONG_MAX;
>>   		pgdat_resize_unlock(pgdat, &flags);
>> -		return true;
>> +		return false;
>>   	}
>>   
>>   	/*
> 
> The one change I might make to this would be to do:
> 	return first_deferred_pfn != ULONG_MAX;
> 
> That way in the event the previous caller did free up the last of the
> pages and empty the zone just before we got here then we will try one more
> time. Otherwise if it was already done before we got here we exit.

Thanks for the constructive feedback!

Will send a non-RFC variant soon.


Juergen