Re: [BUG] Crash on x86_32 for: mm: page_alloc: avoid merging non-fallbackable pageblocks with others

[Vlastimil Babka <vbabka@suse.cz>]

On 3/30/22 23:43, Zi Yan wrote:
> On 30 Mar 2022, at 17:25, Zi Yan wrote:
> 
>> On 30 Mar 2022, at 16:53, Steven Rostedt wrote:
>>
>>> On Wed, 30 Mar 2022 16:29:28 -0400
>>> Zi Yan <ziy@nvidia.com> wrote:
>>>
>>>> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
>>>> index bdc8f60ae462..83a90e2973b7 100644
>>>> --- a/mm/page_alloc.c
>>>> +++ b/mm/page_alloc.c
>>>> @@ -1108,6 +1108,8 @@ static inline void __free_one_page(struct page *page,
>>>>
>>>>                 buddy_pfn = __find_buddy_pfn(pfn, order);
>>>>                 buddy = page + (buddy_pfn - pfn);
>>>> +               if (!page_is_buddy(page, buddy, order))
>>>> +                       goto done_merging;
>>>>                 buddy_mt = get_pageblock_migratetype(buddy);
>>>>
>>>>                 if (migratetype != buddy_mt
>>>>
>>>
>>> The above did not apply to Linus's tree, nor even the problem commit
>>> (before or after), but I found where the code is, and added it manually.
>>>
>>> It does appear to allow the machine to boot.
>>>
>> I just pulled Linus’s tree and grabbed the diff. Anyway, thanks.
>>
>> I would like to get more understanding of the issue before blindly sending
>> this as a fix.
>>
>> Merge the other thread:
>>>
>>> Not sure if this matters or not, but my kernel command line has:
>>>
>>>   crashkernel=256M
>>>
>>> Could that have caused this to break?
>>
>> Unlikely, 256MB is MAX_ORDER_NR_PAGES aligned (MAX_ORDER is 11 here).
>> __find_buddy_pfn() will not get any buddy_pfn from crashkernel memory
>> region, since that would cross MAX_ORDER_NR_PAGES boundary.
>>
>> page_is_buddy() checks page_is_guard(buddy), PageBuddy(buddy),
>> buddy_order(buddy), and page_zone_id(buddy), where page_is_guard(buddy)
>> is always false since CONFIG_DEBUG_PAGEALLOC is not set in your config.
>> So either PageBuddy(buddy) is false, buddy_order(buddy) != order,
>> or page_zone_id(buddy) is not the same as page_zone_id(page).
>>
>> Do you mind adding the following code right before my fix code above
>> and provide a complete boot log? I would like to understand what
>> went wrong. Thanks.
>>
>> pr_info("buddy_pfn: %lx, PageBuddy: %d, buddy_order: %d (vs %d), page_zone_id: %d (vs %d)\n",
>>         buddy_pfn, PageBuddy(buddy), buddy_order(buddy), order, page_zone_id(buddy),
>>         page_zone_id(page));
>>
>>
> 
> This seems to be a bug in the original code too.
> But "if (unlikely(has_isolate_pageblock(zone)))" is too rare to trigger it.
> I do not see how having isolated pageblocks in a zone could get us away
> from checking page_is_buddy().

IIRC the assumption was that pageblock bitmaps would always exist
withing MAX_ORDER blocks. But here we are still under mem_init() where
has_isolate_pageblock() couldn't happen. And the assumption could have
been silently broken by subsequent memory init changes.

> --
> Best Regards,
> Yan, Zi