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[2003:d8:2f45:5500:8267:647f:4209:dedd]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-442cd3aecb0sm66649365e9.28.2025.05.09.02.38.05 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 09 May 2025 02:38:05 -0700 (PDT) Message-ID: Date: Fri, 9 May 2025 11:38:04 +0200 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: Memory hotplug locking issue: Useless (?) zone span seqlock To: Mathieu Desnoyers , Andrew Morton , Oscar Salvador Cc: Michal Hocko , Anshuman Khandual , Vlastimil Babka , Pavel Tatashin , Linus Torvalds , linux-kernel , linux-mm References: <8a4a5d6e-d2cf-420d-91f3-2797618e7255@efficios.com> From: David Hildenbrand Content-Language: en-US Autocrypt: addr=david@redhat.com; keydata= xsFNBFXLn5EBEAC+zYvAFJxCBY9Tr1xZgcESmxVNI/0ffzE/ZQOiHJl6mGkmA1R7/uUpiCjJ dBrn+lhhOYjjNefFQou6478faXE6o2AhmebqT4KiQoUQFV4R7y1KMEKoSyy8hQaK1umALTdL QZLQMzNE74ap+GDK0wnacPQFpcG1AE9RMq3aeErY5tujekBS32jfC/7AnH7I0v1v1TbbK3Gp XNeiN4QroO+5qaSr0ID2sz5jtBLRb15RMre27E1ImpaIv2Jw8NJgW0k/D1RyKCwaTsgRdwuK Kx/Y91XuSBdz0uOyU/S8kM1+ag0wvsGlpBVxRR/xw/E8M7TEwuCZQArqqTCmkG6HGcXFT0V9 PXFNNgV5jXMQRwU0O/ztJIQqsE5LsUomE//bLwzj9IVsaQpKDqW6TAPjcdBDPLHvriq7kGjt WhVhdl0qEYB8lkBEU7V2Yb+SYhmhpDrti9Fq1EsmhiHSkxJcGREoMK/63r9WLZYI3+4W2rAc UucZa4OT27U5ZISjNg3Ev0rxU5UH2/pT4wJCfxwocmqaRr6UYmrtZmND89X0KigoFD/XSeVv jwBRNjPAubK9/k5NoRrYqztM9W6sJqrH8+UWZ1Idd/DdmogJh0gNC0+N42Za9yBRURfIdKSb B3JfpUqcWwE7vUaYrHG1nw54pLUoPG6sAA7Mehl3nd4pZUALHwARAQABzSREYXZpZCBIaWxk ZW5icmFuZCA8ZGF2aWRAcmVkaGF0LmNvbT7CwZgEEwEIAEICGwMGCwkIBwMCBhUIAgkKCwQW AgMBAh4BAheAAhkBFiEEG9nKrXNcTDpGDfzKTd4Q9wD/g1oFAl8Ox4kFCRKpKXgACgkQTd4Q 9wD/g1oHcA//a6Tj7SBNjFNM1iNhWUo1lxAja0lpSodSnB2g4FCZ4R61SBR4l/psBL73xktp rDHrx4aSpwkRP6Epu6mLvhlfjmkRG4OynJ5HG1gfv7RJJfnUdUM1z5kdS8JBrOhMJS2c/gPf wv1TGRq2XdMPnfY2o0CxRqpcLkx4vBODvJGl2mQyJF/gPepdDfcT8/PY9BJ7FL6Hrq1gnAo4 3Iv9qV0JiT2wmZciNyYQhmA1V6dyTRiQ4YAc31zOo2IM+xisPzeSHgw3ONY/XhYvfZ9r7W1l pNQdc2G+o4Di9NPFHQQhDw3YTRR1opJaTlRDzxYxzU6ZnUUBghxt9cwUWTpfCktkMZiPSDGd KgQBjnweV2jw9UOTxjb4LXqDjmSNkjDdQUOU69jGMUXgihvo4zhYcMX8F5gWdRtMR7DzW/YE BgVcyxNkMIXoY1aYj6npHYiNQesQlqjU6azjbH70/SXKM5tNRplgW8TNprMDuntdvV9wNkFs 9TyM02V5aWxFfI42+aivc4KEw69SE9KXwC7FSf5wXzuTot97N9Phj/Z3+jx443jo2NR34XgF 89cct7wJMjOF7bBefo0fPPZQuIma0Zym71cP61OP/i11ahNye6HGKfxGCOcs5wW9kRQEk8P9 M/k2wt3mt/fCQnuP/mWutNPt95w9wSsUyATLmtNrwccz63XOwU0EVcufkQEQAOfX3n0g0fZz Bgm/S2zF/kxQKCEKP8ID+Vz8sy2GpDvveBq4H2Y34XWsT1zLJdvqPI4af4ZSMxuerWjXbVWb T6d4odQIG0fKx4F8NccDqbgHeZRNajXeeJ3R7gAzvWvQNLz4piHrO/B4tf8svmRBL0ZB5P5A 2uhdwLU3NZuK22zpNn4is87BPWF8HhY0L5fafgDMOqnf4guJVJPYNPhUFzXUbPqOKOkL8ojk CXxkOFHAbjstSK5Ca3fKquY3rdX3DNo+EL7FvAiw1mUtS+5GeYE+RMnDCsVFm/C7kY8c2d0G NWkB9pJM5+mnIoFNxy7YBcldYATVeOHoY4LyaUWNnAvFYWp08dHWfZo9WCiJMuTfgtH9tc75 7QanMVdPt6fDK8UUXIBLQ2TWr/sQKE9xtFuEmoQGlE1l6bGaDnnMLcYu+Asp3kDT0w4zYGsx 5r6XQVRH4+5N6eHZiaeYtFOujp5n+pjBaQK7wUUjDilPQ5QMzIuCL4YjVoylWiBNknvQWBXS lQCWmavOT9sttGQXdPCC5ynI+1ymZC1ORZKANLnRAb0NH/UCzcsstw2TAkFnMEbo9Zu9w7Kv AxBQXWeXhJI9XQssfrf4Gusdqx8nPEpfOqCtbbwJMATbHyqLt7/oz/5deGuwxgb65pWIzufa N7eop7uh+6bezi+rugUI+w6DABEBAAHCwXwEGAEIACYCGwwWIQQb2cqtc1xMOkYN/MpN3hD3 AP+DWgUCXw7HsgUJEqkpoQAKCRBN3hD3AP+DWrrpD/4qS3dyVRxDcDHIlmguXjC1Q5tZTwNB boaBTPHSy/Nksu0eY7x6HfQJ3xajVH32Ms6t1trDQmPx2iP5+7iDsb7OKAb5eOS8h+BEBDeq 3ecsQDv0fFJOA9ag5O3LLNk+3x3q7e0uo06XMaY7UHS341ozXUUI7wC7iKfoUTv03iO9El5f XpNMx/YrIMduZ2+nd9Di7o5+KIwlb2mAB9sTNHdMrXesX8eBL6T9b+MZJk+mZuPxKNVfEQMQ a5SxUEADIPQTPNvBewdeI80yeOCrN+Zzwy/Mrx9EPeu59Y5vSJOx/z6OUImD/GhX7Xvkt3kq Er5KTrJz3++B6SH9pum9PuoE/k+nntJkNMmQpR4MCBaV/J9gIOPGodDKnjdng+mXliF3Ptu6 3oxc2RCyGzTlxyMwuc2U5Q7KtUNTdDe8T0uE+9b8BLMVQDDfJjqY0VVqSUwImzTDLX9S4g/8 kC4HRcclk8hpyhY2jKGluZO0awwTIMgVEzmTyBphDg/Gx7dZU1Xf8HFuE+UZ5UDHDTnwgv7E th6RC9+WrhDNspZ9fJjKWRbveQgUFCpe1sa77LAw+XFrKmBHXp9ZVIe90RMe2tRL06BGiRZr jPrnvUsUUsjRoRNJjKKA/REq+sAnhkNPPZ/NNMjaZ5b8Tovi8C0tmxiCHaQYqj7G2rgnT0kt WNyWQQ== Organization: Red Hat In-Reply-To: <8a4a5d6e-d2cf-420d-91f3-2797618e7255@efficios.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 08.05.25 15:11, Mathieu Desnoyers wrote: > On 2025-05-08 06:45, David Hildenbrand wrote: >> On 07.03.25 21:22, Mathieu Desnoyers wrote: >>> I'm currently perfecting my understanding of the mm code and reviewing >>> pieces of it as I go, and stumbled on this: >>> >>> commit 27cacaad16c5 ("mm,memory_hotplug: drop unneeded locking") >>> >>> This commit removes all users of zone_span_writelock(), thus making >>> the inline useless, but leaves the now useless >>> zone_span_seqbegin()/zone_span_seqretry() in place within >>> page_outside_zone_boundaries(). >>> >>> So I'm confused. What's going on ? >>> >>> And if this commit got things very wrong when removing the >>> seqlock, I wonder if there are cases where its partial >>> pgdat_resize_lock() removal can be an issue as well. >> >> I stumbled over that myself recently as well. I think I mentioned in the >> past that we should just store >> >> start_pfn + end_pfn >> >> instead of >> >> start_pfn + nr_pages >> >> >> Then, concurrent resizing could happen (and we could atomically read >> start_pfn / end_pfn). >> >> Right now, when adjusting start_pfn, we always also have to adjust >> nr_pages. A concurrent reader calculating end_pfn manually could see >> some crappy result. >> >> Having that said, I am not aware of issues in that area, but it all >> looks like only a partial cleanup to me. > > I wonder if all callers to zone_spans_pfn() prevent concurrent modification > of the zone start_pfn and nr_pages ? I think compaction does not protect against that (e.g., no mem_hotplug_begin()). My understanding was that compaction can mostly tolerate it (e.g., recheck pfn_to_online_page() + page_zone() check) > > For instance set_pfnblock_flags_mask() (called by set_pageblock_migratetype) > does: > > VM_BUG_ON_PAGE(!zone_spans_pfn(page_zone(page), pfn), page); > Yes, that might indeed trigger in some weird races. > If we look at zone_spans_pfn(): > > static inline unsigned long zone_end_pfn(const struct zone *zone) > { > return zone->zone_start_pfn + zone->spanned_pages; > } > > static inline bool zone_spans_pfn(const struct zone *zone, unsigned long pfn) > { > return zone->zone_start_pfn <= pfn && pfn < zone_end_pfn(zone); > } > > A concurrent updater which shrinks a zone by moving its start would have > to increment zone_start_pfn *and* decrement spanned_pages. If this happens > concurrently with the loads from zone_spans_pfn(), then it can observe > an intermediate state (only nr pages reduced or only zone start moved). > The first scenario at least can lead to false positive VM_BUG_ON(). Jup. > > Likewise if the updater expands the zone by moving its start left. If > the observer loads an updated start pfn without observing the nr pages > update, it can lead to false positive VM_BUG_ON(). > > I notice that zone_intersects() also uses zone_end_pfn(). It is used for > instance by default_kernel_zone_for_pfn() without locks. In this case, > reading both nr pages and start pfn concurrently with update could cause > a false-positive match, for instance if the start of the range is moved > but the nr pages prior value is loaded (concurrent shrink). This could > match a zone outside of the function parameter range. > > Another reader of those fields is update_pgdat_span(), which appears to > be called only from remove_pfn_range_from_zone with mem_hotplug_lock > held in write mode. So that one should be fine. > > AFAIU, updates to nr pages and zone start pfn are done by: > > - remove_pfn_range_from_zone (AFAIU always called with mem_hotplug_lock > in write mode), > - shrink_zone_span (called from remove_pfn_range_from_zone), > - resize_zone_range (__meminit function), called from move_pfn_range_to_zone() > also called with mem_hotplug_lock in write mode. Yes, I refactored that at some point. mem_hotplug_begin() could protect against it, but we really don't want to grab that all over the place. > > So I think your idea of keeping track of both zone_start_pfn and zone_end_pfn > would solve this specific issue, however we'd have to carefully consider what > happens to users of spanned_pages (e.g. zone_is_empty()) callers, because it > would then require to calculate the spanned_pages from end - start, which > then can have similar issues wrt atomicity against concurrent updates. We could easily just store both things (spanned pages + end_pfn), such that we can read both atomically. -- Cheers, David / dhildenb