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([2a09:80c0:192:0:5dac:bf3d:c41:c3e7]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-37cd565e767sm8559871f8f.38.2024.09.30.02.25.08 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 30 Sep 2024 02:25:09 -0700 (PDT) Message-ID: Date: Mon, 30 Sep 2024 11:25:08 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v3 1/5] mm: memory_hotplug: remove head variable in do_migrate_range() To: Miaohe Lin , Kefeng Wang , Matthew Wilcox Cc: Andrew Morton , Oscar Salvador , Naoya Horiguchi , linux-mm@kvack.org, dan.carpenter@linaro.org, Jonathan Cameron References: <20240827114728.3212578-1-wangkefeng.wang@huawei.com> <20240827114728.3212578-2-wangkefeng.wang@huawei.com> <20a75b57-12a6-468f-bd7c-0aeb2f259228@redhat.com> <170546a8-e442-91e9-31e8-60a91018172a@huawei.com> <841eb150-fac6-461e-808f-e6ae607c7d81@huawei.com> <986bca05-7fd9-49ac-9129-934f31c28af6@huawei.com> From: David Hildenbrand 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: <986bca05-7fd9-49ac-9129-934f31c28af6@huawei.com> X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Rspam-User: X-Stat-Signature: jiybfngmyt6ggwfpwjpzeunh6oq78hm8 X-Rspamd-Queue-Id: 6460A40003 X-Rspamd-Server: rspam11 X-HE-Tag: 1727688313-530878 X-HE-Meta: U2FsdGVkX19gwCqwmaEimJfAqBG7LihMQTzKFSAyXAapSMQnvA7H8XqG/mINs01LH29DuKxRLwb3tETDGSsuTgLN+1W3I6e5siHVHWThTl8UMXPHlhdSwmiVKlAEcN26iJfriHzXjYU0oqF4cdsV9VABIgrlW526Sgtbxy91698Qz+Pq30ADB/320PJODu+SCkb0AkTCiSP37BMQTWWCCEzkTTh57B+yQDfuRvEXFy+ed9xADMfCbGSdtCLS7dBCM9fGGWPlZQQEs5DLFYeFE8oPRsxVBWiTmTId+pBvlhzGiXlReE9FK7BANPALbFdgQsFpCKYomWNw3YKYevEK6Ukk56kaKvw4P06xZqBXxR2czdYnPedLofhRd4Sal1nicBbrjR6EOpUTGR+irJBueACw8R3mi6ZY+yUHRncjq3JLqw9yZFdJCWD51jmmPKxbBoRAq0PZl6Wk33nXIf4ebGTGV2WLpfXRxJjRvWlK97jLkw4V0L6uND45Kdp57jP8O3GHIU4lo4/CYwT39DRowGdMAZusnNj09HO16mSP/fD7/dVxUMWlpW0HPo4aRqI87zbv6MT1+IxXvibScYT81+v/EJM1YQ6c6Ubz+AkPE9NUmd1/mXQNUHnCQ48Mo+L79PMWlHFhcuNUW7WRHMloqb1djiQs5L8VrjZYBhuKllt20dM5Mopf2yty7VIgTqhgjUKvG6Q/YeYRDw9BU64jmsOUcm4HT8s4DFOk6/SFTGWx9vLrv0NXMnyNaTJ3UaB3o7GRFK33vY4cafeWHjSPxe0c4HoZLBl/sQJARV/AnKVwcTaO5mXjpcDLfags8ARGf3pEY+KClp3ROXKeQ/t5GUwaeR26o1b3OWn4YlNXHHHwpE+woZx98CZBuzyuXOMPdpuDEn/MYpExM1FYLRAT3/nZGd2+EwmfJgP+799wxvGcYpwAMrQLAWJ6vG5GZC0Ipih1HRloH+wk6KpccU7 Vbht1GC8 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 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: List-Subscribe: List-Unsubscribe: On 29.09.24 04:19, Miaohe Lin wrote: > On 2024/9/29 10:04, Kefeng Wang wrote: >> >> >> On 2024/9/29 9:16, Miaohe Lin wrote: >>> On 2024/9/28 16:39, David Hildenbrand wrote: >>>> On 28.09.24 10:34, David Hildenbrand wrote: >>>>> On 28.09.24 06:55, Matthew Wilcox wrote: >>>>>> On Tue, Aug 27, 2024 at 07:47:24PM +0800, Kefeng Wang wrote: >>>>>>> Directly use a folio for HugeTLB and THP when calculate the next pfn, then >>>>>>> remove unused head variable. >>>>>> >>>>>> I just noticed this got merged.  You're going to hit BUG_ON with it. >>>>>> >>>>>>> -        if (PageHuge(page)) { >>>>>>> -            pfn = page_to_pfn(head) + compound_nr(head) - 1; >>>>>>> -            isolate_hugetlb(folio, &source); >>>>>>> -            continue; >>>>>>> -        } else if (PageTransHuge(page)) >>>>>>> -            pfn = page_to_pfn(head) + thp_nr_pages(page) - 1; >>>>>>> +        /* >>>>>>> +         * No reference or lock is held on the folio, so it might >>>>>>> +         * be modified concurrently (e.g. split).  As such, >>>>>>> +         * folio_nr_pages() may read garbage.  This is fine as the outer >>>>>>> +         * loop will revisit the split folio later. >>>>>>> +         */ >>>>>>> +        if (folio_test_large(folio)) { >>>>>> >>>>>> But it's not fine.  Look at the implementation of folio_test_large(): >>>>>> >>>>>> static inline bool folio_test_large(const struct folio *folio) >>>>>> { >>>>>>            return folio_test_head(folio); >>>>>> } >>>>>> >>>>>> That's going to be provided by: >>>>>> >>>>>> #define FOLIO_TEST_FLAG(name, page)                                     \ >>>>>> static __always_inline bool folio_test_##name(const struct folio *folio) \ >>>>>> { return test_bit(PG_##name, const_folio_flags(folio, page)); } >>>>>> >>>>>> and here's the BUG: >>>>>> >>>>>> static const unsigned long *const_folio_flags(const struct folio *folio, >>>>>>                    unsigned n) >>>>>> { >>>>>>            const struct page *page = &folio->page; >>>>>> >>>>>>            VM_BUG_ON_PGFLAGS(PageTail(page), page); >>>>>>            VM_BUG_ON_PGFLAGS(n > 0 && !test_bit(PG_head, &page->flags), page); >>>>>>            return &page[n].flags; >>>>>> } >>>>>> >>>>>> (this page can be transformed from a head page to a tail page because, >>>>>> as the comment notes, we don't hold a reference. >>>>>> >>>>>> Please back this out. >>>>> >>>>> Should we generalize the approach in dump_folio() to locally copy a >>>>> folio, so we can safely perform checks before deciding whether we want >>>>> to try grabbing a reference on the real folio (if it's still a folio :) )? >>>>> >>>> >>>> Oh, and I forgot: isn't the existing code already racy? >>>> >>>> PageTransHuge() -> VM_BUG_ON_PAGE(PageTail(page), page); >> >> Yes, in v1[1], I asked same question for existing code for PageTransHuge(page), >> >>   "If the page is a tail page, we will BUG_ON(DEBUG_VM enabled) here, >>    but it seems that we don't guarantee the page won't be a tail page." >> >> >> we could delay the calculation after we got a ref, but the traversal of pfn may slow down a little if hint a tail pfn, is it acceptable? >> >> --- a/mm/memory_hotplug.c >> +++ b/mm/memory_hotplug.c >> @@ -1786,15 +1786,6 @@ static void do_migrate_range(unsigned long start_pfn, unsigned long end_pfn) >>                 page = pfn_to_page(pfn); >>                 folio = page_folio(page); >> >> -               /* >> -                * No reference or lock is held on the folio, so it might >> -                * be modified concurrently (e.g. split).  As such, >> -                * folio_nr_pages() may read garbage.  This is fine as the outer >> -                * loop will revisit the split folio later. >> -                */ >> -               if (folio_test_large(folio)) >> -                       pfn = folio_pfn(folio) + folio_nr_pages(folio) - 1; >> - >>                 /* >>                  * HWPoison pages have elevated reference counts so the migration would >>                  * fail on them. It also doesn't make any sense to migrate them in the >> @@ -1807,6 +1798,8 @@ static void do_migrate_range(unsigned long start_pfn, unsigned long end_pfn) >>                                 folio_isolate_lru(folio); >>                         if (folio_mapped(folio)) >>                                 unmap_poisoned_folio(folio, TTU_IGNORE_MLOCK); >> +                       if (folio_test_large(folio)) >> +                               pfn = folio_pfn(folio) + folio_nr_pages(folio) - 1; >>                         continue; >>                 } >> >> @@ -1823,6 +1816,9 @@ static void do_migrate_range(unsigned long start_pfn, unsigned long end_pfn) >>                                 dump_page(page, "isolation failed"); >>                         } >>                 } >> + >> +               if (folio_test_large(folio)) >> +                       pfn = folio_pfn(folio) + folio_nr_pages(folio) - 1; >>  put_folio: >>                 folio_put(folio); >>         } >> >> >>> >>> do_migrate_range is called after start_isolate_page_range(). So a page might not be able to >>> transform from a head page to a tail page as it's isolated? >> start_isolate_page_range() is only isolate free pages, so maybe irrelevant. > > A page transform from a head page to a tail page should through the below steps: > 1. The compound page is freed into buddy. > 2. It's merged into larger order in buddy. > 3. It's allocated as a larger order compound page. > > Since it is isolated, I think step 2 or 3 cannot happen. Or am I miss something? By isolated, you mean that the pageblock is isolated, and all free pages are in the MIGRATE_ISOLATE buddy list. Nice observation. Indeed, a tail page could become a head page (concurrent split is possible), but a head page should not become a tail for the reason you mention. Even mm/page_reporting.c will skip isolated pageblocks. I wonder if there are some corner cases, but nothing comes to mind that would perform compound allocations from the MIGRATE_ISOLATE list. -- Cheers, David / dhildenb