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

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Message-ID: <96b7c46d-8af1-4841-c97e-694abcf3b28b@redhat.com>
Date:   Tue, 7 Sep 2021 11:52:51 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Miaohe Lin <linmiaohe@...wei.com>, akpm@...ux-foundation.org
Cc:     vbabka@...e.cz, 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 03:46, Miaohe Lin wrote:
> 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@...wei.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@...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);
> 
> Should this be VM_BUG_ON(!isolated_page) ?
> 

Ehm, yes :)

-- 
Thanks,

David / dhildenb