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[2003:cb:c739:2400:78ac:64bb:a39e:2578]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-36792881856sm174630f8f.6.2024.07.02.14.29.30 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 02 Jul 2024 14:29:30 -0700 (PDT) Message-ID: <15f16b06-e129-49af-ae11-62ff6615d229@redhat.com> Date: Tue, 2 Jul 2024 23:29:29 +0200 Precedence: bulk X-Mailing-List: stable@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v1] mm: Fix khugepaged activation policy To: Ryan Roberts , Andrew Morton , Jonathan Corbet , Barry Song , Baolin Wang , Lance Yang , Yang Shi Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, stable@vger.kernel.org References: <20240702144617.2291480-1-ryan.roberts@arm.com> <686d3f32-45b0-4dd5-8954-e75748b9c946@redhat.com> <46fb005b-30e4-4340-b1e2-8789833f952c@arm.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: <46fb005b-30e4-4340-b1e2-8789833f952c@arm.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit On 02.07.24 18:33, Ryan Roberts wrote: > On 02/07/2024 16:38, David Hildenbrand wrote: >> On 02.07.24 17:29, Ryan Roberts wrote: >>> On 02/07/2024 15:57, David Hildenbrand wrote: >>>> On 02.07.24 16:46, Ryan Roberts wrote: >>>>> Since the introduction of mTHP, the docuementation has stated that >>>>> khugepaged would be enabled when any mTHP size is enabled, and disabled >>>>> when all mTHP sizes are disabled. There are 2 problems with this; 1. >>>>> this is not what was implemented by the code and 2. this is not the >>>>> desirable behavior. >>>>> >>>>> Desirable behavior is for khugepaged to be enabled when any PMD-sized >>>>> THP is enabled, anon or file. (Note that file THP is still controlled by >>>>> the top-level control so we must always consider that, as well as the >>>>> PMD-size mTHP control for anon). khugepaged only supports collapsing to >>>>> PMD-sized THP so there is no value in enabling it when PMD-sized THP is >>>>> disabled. So let's change the code and documentation to reflect this >>>>> policy. >>>>> >>>>> Further, per-size enabled control modification events were not >>>>> previously forwarded to khugepaged to give it an opportunity to start or >>>>> stop. Consequently the following was resulting in khugepaged eroneously >>>>> not being activated: >>>>> >>>>>     echo never > /sys/kernel/mm/transparent_hugepage/enabled >>>>>     echo always > /sys/kernel/mm/transparent_hugepage/hugepages-2048kB/enabled >>>>> >>>>> Signed-off-by: Ryan Roberts >>>>> Fixes: 3485b88390b0 ("mm: thp: introduce multi-size THP sysfs interface") >>>>> Closes: >>>>> https://lore.kernel.org/linux-mm/7a0bbe69-1e3d-4263-b206-da007791a5c4@redhat.com/ >>>>> Cc: stable@vger.kernel.org >>>>> --- >>>>> >>>>> Hi All, >>>>> >>>>> Applies on top of today's mm-unstable (9bb8753acdd8). No regressions >>>>> observed in >>>>> mm selftests. >>>>> >>>>> When fixing this I also noticed that khugepaged doesn't get (and never has >>>>> been) >>>>> activated/deactivated by `shmem_enabled=`. I'm not sure if khugepaged knows how >>>>> to collapse shmem - perhaps it should be activated in this case? >>>>> >>>> >>>> Call me confused. >>>> >>>> khugepaged_scan_mm_slot() and madvise_collapse() only all >>>> hpage_collapse_scan_file() with ... IS_ENABLED(CONFIG_SHMEM) ? >>> >>> Looks like khugepaged_scan_mm_slot() was converted from: >>> >>>    if (shmem_file(vma->vm_file)) { >>> >>> to: >>> >>>    if (IS_ENABLED(CONFIG_SHMEM) && vma->vm_file) { > > CONFIG_READ_ONLY_THP_FOR_FS depends on CONFIG_SHMEM in Kconfig, so I think this is all correct/safe. Although I'm not really sure what the need for the dependency is. Right. It's likely some historical artifact ... [...] > > I'll do a v2 with this addition if you agree? > > static inline bool hugepage_pmd_enabled(void) > { > /* > * We cover both the anon and the file-backed case here; for > * file-backed, we must return true if globally enabled, regardless of > * the anon pmd size control status. > */ > return (IS_ENABLED(CONFIG_READ_ONLY_THP_FOR_FS) && hugepage_global_enabled()) || > test_bit(PMD_ORDER, &huge_anon_orders_always) || > test_bit(PMD_ORDER, &huge_anon_orders_madvise) || > (test_bit(PMD_ORDER, &huge_anon_orders_inherit) && hugepage_global_enabled()); > } > I think this looks good :) >>> >>> But back to my original question, I think hugepage_pmd_enabled() should also be >>> explicitly checking the appropriate shmem_enabled controls and ORing in the >>> result? Otherwise in a situation where only shmem is THP enabled (and file/anon >>> THP is disabled) khugepaged won't run. >> >> I think so. > > I'll do this part as a separate change since it's fixing a separate bug. Makes sense. > >> >>> >>>> >>>> The thp_vma_allowable_order() check tests if we are allowed to collapse a >>>> PMD_ORDER in that VMA. >>> >>> I don't follow the relevance of this statement. >> >> On whatever VMA we indicate true, we will try to collapse in khugepaged. >> Regarding the question, what khugepaged will try to collapse. > > Oh I see. Yes agreed. But we don't have a VMA when enabling/disabling khugepaged. We just want to know "are there vmas for which khugepaged would attempt to collapse to PMD size?" Yes, only to give you a pointer on which VMAs khugepaged will be willing to work with. -- Cheers, David / dhildenb