linux-kernel - Re: [PATCH] mm/page_isolation: fix potential missing call to unset_migratetype

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Date:   Mon, 6 Sep 2021 17:38:58 +0800
From:   Miaohe Lin <linmiaohe@...wei.com>
To:     David Hildenbrand <david@...hat.com>, <akpm@...ux-foundation.org>
CC:     <mhocko@...e.com>, <vbabka@...e.cz>, <linux-mm@...ck.org>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mm/page_isolation: fix potential missing call to
 unset_migratetype_isolate()

On 2021/9/6 17:27, David Hildenbrand wrote:
> On 04.09.21 11:20, Miaohe Lin wrote:
>> In start_isolate_page_range() undo path, pfn_to_online_page() just checks
>> the first pfn in a pageblock while __first_valid_page() will traverse the
>> pageblock until the first online pfn is found. So we may miss the call to
>> unset_migratetype_isolate() in undo path and pages will remain isolated
>> unexpectedly. Fix this by calling undo_isolate_page_range() and this will
>> also help to remove some duplicated codes.
>>
>> Fixes: 2ce13640b3f4 ("mm: __first_valid_page skip over offline pages")
> 
> While that is true, we shouldn't ever trigger, neither via cma, virtio-mem nor memory offlining, because essentially all operate on MAX_ORDER -1 -aligned ranges without memory holes.

I think this should never trigger too. It's a theoretical issue. So is the Fixes tag necessary ?

> 
>> Signed-off-by: Miaohe Lin <linmiaohe@...wei.com>
>> ---
>>   mm/page_isolation.c | 9 +--------
>>   1 file changed, 1 insertion(+), 8 deletions(-)
>>
>> diff --git a/mm/page_isolation.c b/mm/page_isolation.c
>> index 471e3a13b541..9bb562d5d194 100644
>> --- a/mm/page_isolation.c
>> +++ b/mm/page_isolation.c
>> @@ -202,14 +202,7 @@ int start_isolate_page_range(unsigned long start_pfn, unsigned long end_pfn,
>>       }
>>       return 0;
>>   undo:
>> -    for (pfn = start_pfn;
>> -         pfn < undo_pfn;
>> -         pfn += pageblock_nr_pages) {
>> -        struct page *page = pfn_to_online_page(pfn);
>> -        if (!page)
>> -            continue;
>> -        unset_migratetype_isolate(page, migratetype);
>> -    }
>> +    undo_isolate_page_range(start_pfn, undo_pfn, migratetype);
>>   
> 
> It'd be even cleaner to drop the label completely and call it from the single callsite. We can even avoid undo_pfn ...
> 
> if (page && set_migratetype_isolate(page, migratetype, flags)) {
>     undo_isolate_page_range(start_pfn, pfn, migratetype);
>     return -EBUSY;
> }
> 

Looks much better. Will do it later. Many thanks. :)

> If pfn == start_pfn, undo_isolate_page_range() will simply do nothing.
> 
>>       return -EBUSY;
>>   }
>>
>