linux-kernel - Re: [PATCH] mm/page_alloc: consider pfn holes after pfn_valid() in __pageblock_pfn_to

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Message-ID: <2ef52fb4-ba74-a23e-4f5b-d4725c9fe289@linux.alibaba.com>
Date:   Wed, 19 Apr 2023 14:47:39 +0800
From:   Baolin Wang <baolin.wang@...ux.alibaba.com>
To:     Vlastimil Babka <vbabka@...e.cz>,
        David Hildenbrand <david@...hat.com>, akpm@...ux-foundation.org
Cc:     mgorman@...hsingularity.net, mhocko@...e.com, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm/page_alloc: consider pfn holes after pfn_valid() in
 __pageblock_pfn_to_page()



On 4/14/2023 11:07 PM, Vlastimil Babka wrote:
> On 4/12/23 14:16, Baolin Wang wrote:
>>
>>
>> On 4/12/2023 7:25 PM, David Hildenbrand wrote:
>>> On 12.04.23 12:45, 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.
>>>
>>> pageblocks are supposed to fall into a single memory section, so in mos > cases, if the start is online, so is the end.
>>
>> Yes, the granularity of memory hotplug is a mem_section.
>>
>> However, suppose the pageblock order is MAX_ORDER-1, and the size of a
>> sub-section is 2M, that means a pageblock will fall into 2 sub
>> mem-section, and if there is a hole in the zone, that means the 2nd sub
>> mem-section can be invalid without setting subsection_map bitmap.
> 
> Can that really happen? I think the buddy merging in __free_one_page() would
> trip on that?

I do not think so IIUC. The hole pfns will not free to buddy system, and 
the buddy system did not change the ms->usage->subsection_map of the 
hole pfns, which indicates the hole pfns are invalid.