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[2003:cb:c726:6100:20f2:6848:5b74:ca82]) by smtp.gmail.com with ESMTPSA id je12-20020a05600c1f8c00b004183edc31adsm34757800wmb.44.2024.04.26.07.39.05 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 26 Apr 2024 07:39:06 -0700 (PDT) Message-ID: <9bf62e97-dfdd-4537-8fb0-b5f293856f59@redhat.com> Date: Fri, 26 Apr 2024 16:39:05 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [RFC v2 0/3] iommu/intel: Free empty page tables on unmaps To: Pasha Tatashin Cc: akpm@linux-foundation.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, rientjes@google.com, dwmw2@infradead.org, baolu.lu@linux.intel.com, joro@8bytes.org, will@kernel.org, robin.murphy@arm.com, iommu@lists.linux.dev, Matthew Wilcox References: <20240426034323.417219-1-pasha.tatashin@soleen.com> <2daf168e-e2b2-4b19-9b39-d58b358c8cd9@redhat.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: 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-Rspamd-Server: rspam08 X-Rspamd-Queue-Id: F3559C0030 X-Stat-Signature: f36pccdzt8fju7qxhddqggxzre5ohamz X-HE-Tag: 1714142351-381651 X-HE-Meta: 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 iYcoeuCo 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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 26.04.24 15:49, Pasha Tatashin wrote: > On Fri, Apr 26, 2024 at 2:42 AM David Hildenbrand wrote: >> >> On 26.04.24 05:43, Pasha Tatashin wrote: >>> Changelog >>> ================================================================ >>> v2: Use mapcount instead of refcount >>> Synchronized with IOMMU Observability changes. >>> ================================================================ >>> >>> This series frees empty page tables on unmaps. It intends to be a >>> low overhead feature. >>> >>> The read-writer lock is used to synchronize page table, but most of >>> time the lock is held is reader. It is held as a writer for short >>> period of time when unmapping a page that is bigger than the current >>> iova request. For all other cases this lock is read-only. >>> >>> page->mapcount is used in order to track number of entries at each page >>> table. >> >> I'm wondering if this will conflict with page_type at some point? We're >> already converting other page table users to ptdesc. CCing Willy. > > Hi David, Hi! > > This contradicts with the following comment in mm_types.h: > * If your page will not be mapped to userspace, you can also use the four > * bytes in the mapcount union, but you must call > page_mapcount_reset() > * before freeing it. I think the documentation is a bit outdated, because we now have page types that are: "For pages that are never mapped to userspace" which includes #define PG_table (we should update that comment, because we're now also using it for hugetlb that can be mapped to user space, which is fine.) Right now, using page->_mapcount would likely still be fine, as long as you cannot end up creating a value that would resemble a type (e.g., PG_offline could be bad). But staring at users of _mapcount and page_mapcount_reset() ... you'd be pretty much the only user of that. mm/zsmalloc.c calls page_mapcount_reset(), and I am not completely sure why ... I can see it touch page->index but not page->_mapcount. Hopefully Willy can comment. -- Cheers, David / dhildenb