Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
Message-ID: <b4615b3c-8217-9f32-39c7-b91c9ec3cccb@suse.cz>
Date:   Tue, 7 Sep 2021 10:08:36 +0200
MIME-Version: 1.0
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101
 Thunderbird/91.0.1
Subject: Re: [PATCH] mm/page_isolation: don't putback unisolated page
Content-Language: en-US
To:     David Hildenbrand <david@redhat.com>,
        Miaohe Lin <linmiaohe@huawei.com>, akpm@linux-foundation.org
Cc:     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:   Vlastimil Babka <vbabka@suse.cz>
In-Reply-To: <c60dc5e2-6f19-3be8-56be-555033cc9ca4@redhat.com>
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit
Precedence: bulk

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@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);
> 
> 
> 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?