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

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Message-ID: <e9ac9692-c126-f78f-a6a2-a88eb986ccb7@suse.cz>
Date:   Fri, 14 Apr 2023 17:07:54 +0200
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Baolin Wang <baolin.wang@...ux.alibaba.com>,
        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 4/12/23 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?