linux-kernel - Re: [PATCH] mm/page_isolation: don't putback unisolated page

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Date:   Tue, 7 Sep 2021 11:56:20 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Vlastimil Babka <vbabka@...e.cz>,
        Miaohe Lin <linmiaohe@...wei.com>, akpm@...ux-foundation.org
Cc:     iamjoonsoo.kim@....com, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm/page_isolation: don't putback unisolated page

On 07.09.21 10:08, Vlastimil Babka wrote:
> On 9/6/21 14:49, 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:
>>>>>
>>>>> Thanks!
>>>>>
>>>>> Reviewed-by: David Hildenbrand <david@...hat.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);
>>
>>
>> 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.
> 
> If this can be handled gracefully, then I'd rather go with VM_WARN_ON.
> Maybe even WARN_ON_ONCE?
> 

I think either VM_BUG_ON() or VM_WARN_ON() -- compiling the runtime 
checks out -- should be good enough.

I'd just go with VM_BUG_ON(), because anybody messing with 
__isolate_free_page() should clearly spot that we expect the current 
handling. But no strong opinion.

-- 
Thanks,

David / dhildenb