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

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Date:   Tue, 7 Sep 2021 10:08:36 +0200
From:   Vlastimil Babka <vbabka@...e.cz>
To:     David Hildenbrand <david@...hat.com>,
        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 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?