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[2003:cb:c741:c700:3db9:94c9:28b0:34f2]) by smtp.gmail.com with ESMTPSA id fj12-20020a05600c0c8c00b0041409db0349sm11999073wmb.48.2024.03.26.09.10.44 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 26 Mar 2024 09:10:44 -0700 (PDT) Message-ID: Date: Tue, 26 Mar 2024 17:10:43 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2] mm: hold PTL from the first PTE while reclaiming a large folio To: Barry Song <21cnbao@gmail.com>, akpm@linux-foundation.org, linux-mm@kvack.org Cc: chrisl@kernel.org, hanchuanhua@oppo.com, hughd@google.com, linux-kernel@vger.kernel.org, mhocko@suse.com, ryan.roberts@arm.com, shy828301@gmail.com, v-songbaohua@oppo.com, wangkefeng.wang@huawei.com, willy@infradead.org, xiang@kernel.org, ying.huang@intel.com, yuzhao@google.com References: <20240306095219.71086-1-21cnbao@gmail.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: <20240306095219.71086-1-21cnbao@gmail.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: 7bit X-Rspamd-Queue-Id: EEF7A100002 X-Rspam-User: X-Stat-Signature: zbccuawws13s1sac6m5958hfypsmfsub X-Rspamd-Server: rspam01 X-HE-Tag: 1711469448-295891 X-HE-Meta: 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 yLXKh3QX 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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 06.03.24 10:52, Barry Song wrote: > From: Barry Song > > Within try_to_unmap_one(), page_vma_mapped_walk() races with other > PTE modifications preceded by pte clear. While iterating over PTEs > of a large folio, it only starts acquiring PTL from the first valid > (present) PTE. PTE modifications can temporarily set PTEs to > pte_none. Consequently, the initial PTEs of a large folio might > be skipped in try_to_unmap_one(). > For example, for an anon folio, if we skip PTE0, we may have PTE0 > which is still present, while PTE1 ~ PTE(nr_pages - 1) are swap > entries after try_to_unmap_one(). > So folio will be still mapped, the folio fails to be reclaimed > and is put back to LRU in this round. > This also breaks up PTEs optimization such as CONT-PTE on this > large folio and may lead to accident folio_split() afterwards. > And since a part of PTEs are now swap entries, accessing those > parts will introduce overhead - do_swap_page. > Although the kernel can withstand all of the above issues, the > situation still seems quite awkward and warrants making it more > ideal. > The same race also occurs with small folios, but they have only > one PTE, thus, it won't be possible for them to be partially > unmapped. > This patch holds PTL from PTE0, allowing us to avoid reading PTE > values that are in the process of being transformed. With stable > PTE values, we can ensure that this large folio is either > completely reclaimed or that all PTEs remain untouched in this > round. > A corner case is that if we hold PTL from PTE0 and most initial > PTEs have been really unmapped before that, we may increase the > duration of holding PTL. Thus we only apply this optimization to > folios which are still entirely mapped (not in deferred_split > list). > > Cc: Hugh Dickins > Signed-off-by: Barry Song > --- > v2: > * Refine commit message and code comment after reading all comments > from Ryan and David, thanks! > * Avoid increasing the duration of PTL by applying optimization > on folios not in deferred_split_list with respect to Ying's > comment, thanks! > > mm/vmscan.c | 12 ++++++++++++ > 1 file changed, 12 insertions(+) > > diff --git a/mm/vmscan.c b/mm/vmscan.c > index 0b888a2afa58..7106741387e8 100644 > --- a/mm/vmscan.c > +++ b/mm/vmscan.c > @@ -1270,6 +1270,18 @@ static unsigned int shrink_folio_list(struct list_head *folio_list, > > if (folio_test_pmd_mappable(folio)) > flags |= TTU_SPLIT_HUGE_PMD; > + /* > + * Without TTU_SYNC, try_to_unmap will only begin to hold PTL > + * from the first present PTE within a large folio. Some initial > + * PTEs might be skipped due to races with parallel PTE writes > + * in which PTEs can be cleared temporarily before being written > + * new present values. This will lead to a large folio is still > + * mapped while some subpages have been partially unmapped after > + * try_to_unmap; TTU_SYNC helps try_to_unmap acquire PTL from the > + * first PTE, eliminating the influence of temporary PTE values. > + */ > + if (folio_test_large(folio) && list_empty(&folio->_deferred_list)) > + flags |= TTU_SYNC; > > try_to_unmap(folio, flags); > if (folio_mapped(folio)) { Hopefully this won't have unexpected performance "surprises". I do wonder if we should really care about the "_deferred_list" optimization here, though, I'd just drop it. In any case Acked-by: David Hildenbrand -- Cheers, David / dhildenb