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Subject: Re: [PATCH] mm/page_isolation: don't putback unisolated page
To:     David Hildenbrand <david@redhat.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>
 <2d06db75-5c26-8fe2-6883-ac99056a9894@redhat.com>
 <b0a2947b-360a-40c2-03e4-f0f67845f4c3@huawei.com>
 <c60dc5e2-6f19-3be8-56be-555033cc9ca4@redhat.com>
From:   Miaohe Lin <linmiaohe@huawei.com>
Message-ID: <57392d12-9a6c-dbb5-3c3e-39ed9ab7c31c@huawei.com>
Date:   Tue, 7 Sep 2021 09:46:57 +0800
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On 2021/9/6 20:49, David Hildenbrand wrote:
> On 06.09.21 14:45, Miaohe Lin wrote:
>> On 2021/9/6 20:11, David Hildenbrand wrote:
>>> 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?
>>
>> I think you're right. __isolate_free_page() should never fail when called from unset_migratetype_isolate()
>> as explained by you. But it might be too fragile to reply on the failure conditions of __isolate_free_page().
>> If that changes, VM_BUG_ON() here might trigger unexpectedly. Or am I just over-worried as failure conditions
>> of __isolate_free_page() can hardly change?
> 
> Maybe
> 
> isolated_page = !!__isolate_free_page(page, order);
> /*
>  * Isolating a free page in an isolated pageblock is expected to always
>  * work as watermarks don't apply here.
>  */
> VM_BUG_ON(isolated_page);

Should this be VM_BUG_ON(!isolated_page) ?

> 
> 
> VM_BUG_ON() allows us to detect any issues when testing. Combined with the comment it tells everybody messing with __isolate_free_page() what we expect in this function.
> 
> In production system, we would handle it gracefully.
> 

Sounds reasonable. Will do it in v2. Many thanks for your suggestion and effort!