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From: Baolin Wang <baolin.wang@linux.alibaba.com>
To: Vlastimil Babka <vbabka@suse.cz>,
	David Hildenbrand <david@redhat.com>,
	akpm@linux-foundation.org
Cc: mgorman@techsingularity.net, mhocko@suse.com, linux-mm@kvack.org,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH] mm/page_alloc: consider pfn holes after pfn_valid() in __pageblock_pfn_to_page()
Date: Wed, 19 Apr 2023 14:47:39 +0800	[thread overview]
Message-ID: <2ef52fb4-ba74-a23e-4f5b-d4725c9fe289@linux.alibaba.com> (raw)
In-Reply-To: <e9ac9692-c126-f78f-a6a2-a88eb986ccb7@suse.cz>



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.


  reply	other threads:[~2023-04-19  6:47 UTC|newest]

Thread overview: 12+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2023-04-12 10:45 [PATCH] mm/page_alloc: consider pfn holes after pfn_valid() in __pageblock_pfn_to_page() Baolin Wang
2023-04-12 11:15 ` Michal Hocko
2023-04-12 12:24   ` Baolin Wang
2023-04-12 11:25 ` David Hildenbrand
2023-04-12 12:16   ` Baolin Wang
2023-04-14 15:07     ` Vlastimil Babka
2023-04-19  6:47       ` Baolin Wang [this message]
2023-04-20  7:22     ` Huang, Ying
2023-04-20  9:11       ` Baolin Wang
2023-04-21  4:21         ` Huang, Ying
2023-04-21  7:13           ` Baolin Wang
2023-04-21  7:44             ` Huang, Ying

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