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Win64; x64; rv:102.0) Gecko/20100101 Thunderbird/102.10.0 Subject: Re: [PATCH] mm/page_alloc: consider pfn holes after pfn_valid() in __pageblock_pfn_to_page() To: Vlastimil Babka , David Hildenbrand , akpm@linux-foundation.org Cc: mgorman@techsingularity.net, mhocko@suse.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <62e231a8f2e50c04dcadc7a0cfaa6dea5ce1ec05.1681296022.git.baolin.wang@linux.alibaba.com> <94bfa3cc-674e-25b0-e7e2-d74c970acef7@redhat.com> From: Baolin Wang In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspamd-Server: rspam07 X-Rspamd-Queue-Id: 736D740019 X-Rspam-User: X-Stat-Signature: u1gakgu4iqxboutse79q8xqisdass63p X-HE-Tag: 1681886862-30484 X-HE-Meta: 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 oPaWZNvq dADH+GjLVjlcCV2uesPK7ILbMAdzzo0V3rzvvWBbCONdSt/z4Z9H4NlOgS4pi+5A7/0nzGS4oychD79aNYzryi1jtsdc8ss/1VyaThW4Pqdf7Docb4CaR2sdAOMmc5dFiDysZiTpWEQms4IQ+aJeEserAGA== X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: 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.