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

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Date:   Wed, 30 Mar 2022 17:25:28 -0400
From:   Zi Yan <ziy@...dia.com>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Mel Gorman <mgorman@...hsingularity.net>,
        David Hildenbrand <david@...hat.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Mike Rapoport <rppt@...ux.ibm.com>,
        Oscar Salvador <osalvador@...e.de>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linux-MM <linux-mm@...ck.org>
Subject: Re: [BUG] Crash on x86_32 for: mm: page_alloc: avoid merging non-fallbackable pageblocks with others

On 30 Mar 2022, at 16:53, Steven Rostedt wrote:

> On Wed, 30 Mar 2022 16:29:28 -0400
> Zi Yan <ziy@...dia.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));


--
Best Regards,
Yan, Zi

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