Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Message-ID: <f30dfee7-526e-07a4-baf6-6be7784beb50@linux.alibaba.com>
Date:   Wed, 12 Apr 2023 20:24:07 +0800
MIME-Version: 1.0
User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:102.0) Gecko/20100101
 Thunderbird/102.9.1
Subject: Re: [PATCH] mm/page_alloc: consider pfn holes after pfn_valid() in
 __pageblock_pfn_to_page()
To:     Michal Hocko <mhocko@suse.com>
Cc:     akpm@linux-foundation.org, mgorman@techsingularity.net,
        vbabka@suse.cz, david@redhat.com, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
References: <62e231a8f2e50c04dcadc7a0cfaa6dea5ce1ec05.1681296022.git.baolin.wang@linux.alibaba.com>
 <ZDaSt8gW7kjfYGx5@dhcp22.suse.cz>
From:   Baolin Wang <baolin.wang@linux.alibaba.com>
In-Reply-To: <ZDaSt8gW7kjfYGx5@dhcp22.suse.cz>
Content-Type: text/plain; charset=UTF-8; format=flowed
Content-Transfer-Encoding: 7bit
Precedence: bulk


On 4/12/2023 7:15 PM, Michal Hocko wrote:
> On Wed 12-04-23 18:45:31, Baolin Wang wrote:
>> Now the __pageblock_pfn_to_page() is used by set_zone_contiguous(),
>> which checks whether the given zone contains holes, and uses pfn_valid()
>> to check if the end pfn is valid. However pfn_valid() can not make sure
>> the end pfn is not a hole if the size of a pageblock is larger than the
>> size of a sub-mem_section, since the struct page getting by pfn_to_page()
>> may represent a hole or an unusable page frame, which may cause incorrect
>> zone contiguous is set.
>>
>> Though another user of pageblock_pfn_to_page() in compaction seems work
>> well now, it is better to avoid scanning or touching these offline pfns.
>> So like commit 2d070eab2e82 ("mm: consider zone which is not fully
>> populated to have holes"), we should also use pfn_to_online_page() for
>> the end pfn to make sure it is a valid pfn with usable page frame.
>> Meanwhile the pfn_valid() for end pfn can be dropped now.
>>
>> Moreover we've already used pfn_to_online_page() for start pfn to make
>> sure it is online and valid, so the pfn_valid() for the start pfn is
>> unnecessary, drop it.
> 
> Is this a theoretical problem or something you have encountered on a
> real machine? Could you provide more details please?

As I replied to David, this is just from code inspection when trying to 
remove the unnecessary pfn_valid() in __pageblock_pfn_to_page().