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

[Zi Yan <ziy@nvidia.com>]

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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().

--
Best Regards,
Yan, Zi

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