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Subject: Re: [PATCH] mm/page_isolation: don't putback unisolated page
From:   David Hildenbrand <david@redhat.com>
To:     Miaohe Lin <linmiaohe@huawei.com>, akpm@linux-foundation.org
Cc:     vbabka@suse.cz, iamjoonsoo.kim@lge.com, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
References: <20210904091839.20270-1-linmiaohe@huawei.com>
 <3b36529f-ab97-ddfe-0407-66f0cd1fd38d@redhat.com>
Organization: Red Hat
Message-ID: <2d06db75-5c26-8fe2-6883-ac99056a9894@redhat.com>
Date:   Mon, 6 Sep 2021 14:11:13 +0200
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On 06.09.21 14:02, David Hildenbrand wrote:
> On 04.09.21 11:18, Miaohe Lin wrote:
>> If __isolate_free_page() failed, due to zone watermark check, the page is
>> still on the free list. But this page will be put back to free list again
>> via __putback_isolated_page() now. This may trigger page->flags checks in
>> __free_one_page() if PageReported is set. Or we will corrupt the free list
>> because list_add() will be called for pages already on another list.
>>
>> Fixes: 3c605096d315 ("mm/page_alloc: restrict max order of merging on isolated pageblock")
>> Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
>> ---
>>    mm/page_isolation.c | 6 ++----
>>    1 file changed, 2 insertions(+), 4 deletions(-)
>>
>> diff --git a/mm/page_isolation.c b/mm/page_isolation.c
>> index 9bb562d5d194..7d70d772525c 100644
>> --- a/mm/page_isolation.c
>> +++ b/mm/page_isolation.c
>> @@ -93,10 +93,8 @@ static void unset_migratetype_isolate(struct page *page, unsigned migratetype)
>>    			buddy_pfn = __find_buddy_pfn(pfn, order);
>>    			buddy = page + (buddy_pfn - pfn);
>>    
>> -			if (!is_migrate_isolate_page(buddy)) {
>> -				__isolate_free_page(page, order);
>> -				isolated_page = true;
>> -			}
>> +			if (!is_migrate_isolate_page(buddy))
>> +				isolated_page = !!__isolate_free_page(page, order);
>>    		}
>>    	}
>>    
>>
> 
> Thanks!
> 
> Reviewed-by: David Hildenbrand <david@redhat.com>
> 

To make the confusion perfect (sorry) :D I tripple-checked:

In unset_migratetype_isolate() we check that 
is_migrate_isolate_page(page) holds, otherwise we return.

We call __isolate_free_page() only for such pages.

__isolate_free_page() won't perform watermark checks on 
is_migrate_isolate().

Consequently, __isolate_free_page() should never fail when called from 
unset_migratetype_isolate()

If that's correct then we  could instead maybe add a VM_BUG_ON() and a 
comment why this can't fail.


Makes sense or am I missing something?

-- 
Thanks,

David / dhildenb