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[2003:cb:c716:7600:ac6:a414:3c04:6f5a]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-3502bbbbde1sm2279992f8f.97.2024.05.09.10.48.46 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 09 May 2024 10:48:47 -0700 (PDT) Message-ID: <23ea6dbd-1d4e-4aeb-900b-646db880cfb6@redhat.com> Date: Thu, 9 May 2024 19:48:46 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 0/8] add mTHP support for anonymous shmem To: Luis Chamberlain , Matthew Wilcox , Christoph Lameter , Christoph Hellwig , Dave Chinner Cc: Daniel Gomez , Baolin Wang , "akpm@linux-foundation.org" , "hughd@google.com" , "ioworker0@gmail.com" , "wangkefeng.wang@huawei.com" , "ying.huang@intel.com" , "21cnbao@gmail.com" <21cnbao@gmail.com>, "ryan.roberts@arm.com" , "shy828301@gmail.com" , "ziy@nvidia.com" , "linux-mm@kvack.org" , "linux-kernel@vger.kernel.org" , Linux FS Devel References: 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: 7bit X-Rspamd-Queue-Id: 15D5E100014 X-Rspam-User: X-Rspamd-Server: rspam03 X-Stat-Signature: zeo375n34sqhc6p6wasrphdoctdgdghe X-HE-Tag: 1715276931-505803 X-HE-Meta: U2FsdGVkX188GrQQo7+2NK/1hapiOQHv72Hy/Z2G9+2j+laCXUjeX9pWhZnkYhZs5GnhMFvQ03jd5s2qWM2AL4ci6lqfHxw1npDNp/FYB2OH0IV76lbfnhuEu/MewJBhkTis6/fo8GmKvkvFCIB2WdAZ801vkBUexcB3hUaOESWJqsK74eEME0+Xu0/hZdoBdIBsRVr33u9MpTNcvI9scSvIemtKK/02uvxohNVMjL83R+DfzkLmRP3qwwJ/QGIuvB1wXnKg9op5YAUdjxSXDMsa0VhjneNYFSZyFZQZW6ZU/BuFxjyqN3tRidJy45ID1wFerHcnqewFXzBlnU2hKQsFdI45MgPzWETWHgYINyUt3TYPN28d8jUkBPFdKZRPp3vgsAKENF5oKygMZFSRvc/s7c5lRCzwNV596nL6kmYrKmxJfVz1XAML4D5pqDMgQYOSfhUtVKoQRquyRYYlO+xZ3hrBkVof6sMPVZElnelPgqmOTTcCzOxDyb75WXNY693GPbMrCMsh+g2Cy1E4fqORazCGzGWXEYilSoEtYbl7pcms9hZ0Xt5c6gNu8vYWsfrkyKqa7ka9Mff86dUMOESp+FaJFadBukNBpPbacxxEpIUNpbN5rSMS8iakAysUVNen9cbF6LypXmZlMnS+Cs5BZ5qoukcVPfkhz95HuFIfVN0tNeSDG3LOlqVkPeeFrS6w/T3vUc2O/3G7RODVITXuiTSDWLiUVsL/7Ab7i9xd+D2VYSpdzsgTdQQICgvxsMBNPIN7luLXFlMXNUBr35SfUOzfXRhbOqGCXLAKz6Tc/UhbXKsJaK0wrRez4Ry38I8WYn7kP0Q8KF3WX+J/zASJZPUfOh/qTFpI1x12U6NbgUFHOkjozeX/SioAr0oNYdccjK/NMdHKlzEq1EnoNsrZuYvoo9BGwk7HPEQSUeXQoKZ+YIiI+mv3C0694IyLIAdL2U93ViYECPBz1O3 hOyQ7GKz 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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 08.05.24 21:23, Luis Chamberlain wrote: > On Wed, May 08, 2024 at 01:58:19PM +0200, David Hildenbrand wrote: >> On 08.05.24 13:39, Daniel Gomez wrote: >>> On Mon, May 06, 2024 at 04:46:24PM +0800, Baolin Wang wrote: >>>> The primary strategy is similar to supporting anonymous mTHP. Introduce >>>> a new interface '/mm/transparent_hugepage/hugepage-XXkb/shmem_enabled', >>>> which can have all the same values as the top-level >>>> '/sys/kernel/mm/transparent_hugepage/shmem_enabled', with adding a new >>>> additional "inherit" option. By default all sizes will be set to "never" >>>> except PMD size, which is set to "inherit". This ensures backward compatibility >>>> with the shmem enabled of the top level, meanwhile also allows independent >>>> control of shmem enabled for each mTHP. >>> >>> I'm trying to understand the adoption of mTHP and how it fits into the adoption >>> of (large) folios that the kernel is moving towards. Can you, or anyone involved >>> here, explain this? How much do they overlap, and can we benefit from having >>> both? Is there any argument against the adoption of large folios here that I >>> might have missed? >> >> mTHP are implemented using large folios, just like traditional PMD-sized THP >> are. >> >> The biggest challenge with memory that cannot be evicted on memory pressure >> to be reclaimed (in contrast to your ordinary files in the pagecache) is >> memory waste, well, and placement of large chunks of memory in general, >> during page faults. >> >> In the worst case (no swap), you allocate a large chunk of memory once and >> it will stick around until freed: no reclaim of that memory. >> >> That's the reason why THP for anonymous memory and SHMEM have toggles to >> manually enable and configure them, in contrast to the pagecache. The same >> was done for mTHP for anonymous memory, and now (anon) shmem follows. >> >> There are plans to have, at some point, have it all working automatically, >> but a lot for that for anonymous memory (and shmem similarly) is still >> missing and unclear. > > Whereas the use for large folios for filesystems is already automatic, > so long as the filesystem supports it. We do this in readahead and write > path already for iomap, we opportunistically use large folios if we can, > otherwise we use smaller folios. > > So a recommended approach by Matthew was to use the readahead and write > path, just as in iomap to determine the size of the folio to use [0]. > The use of large folios would also be automatic and not require any > knobs at all. Yes, I remember discussing that with Willy at some point, including why shmem is unfortunately a bit more "special", because you might not even have a disk backend ("swap") at all where you could easily reclaim memory. In the extreme form, you can consider SHMEM as memory that might be always mlocked, even without the user requiring special mlock limits ... > > The mTHP approach would be growing the "THP" use in filesystems by the > only single filesystem to use THP. Meanwhile use of large folios is already > automatic with the approach taken by iomap. Yes, it's the extension of existing shmem_enabled (that -- I'm afraid -- was added for good reasons). > > We're at a crux where it does beg the question if we should continue to > chug on with tmpfs being special and doing things differently extending > the old THP interface with mTHP, or if it should just use large folios > using the same approach as iomap did. I'm afraid shmem will remain to some degree special. Fortunately it's not alone, hugetlbfs is even more special ;) > > From my perspective the more shared code the better, and the more shared > paths the better. There is a chance to help test swap with large folios > instead of splitting the folios for swap, and that would could be done > first with tmpfs. I have not evaluated the difference in testing or how > we could get the most of shared code if we take a mTHP approach or the > iomap approach for tmpfs, that should be considered. I don't have a clear picture yet of what might be best for ordinary shmem (IOW, not MAP_SHARED|MAP_PRIVATE), and I'm afraid there is no easy answer. As long as we don't end up wasting memory, it's not obviously bad. But some things might be tricky (see my example about large folios stranding in shmem and never being able to be really reclaimed+reused for better purposes) I'll note that mTHP really is just (supposed to be) a user interface to enable the various folio sizes (well, and to expose better per-size stats), not more. From that point of view, it's just a filter. Enable all, and you get the same behavior as you likely would in the pagecache mode. From a shared-code and testing point of view, there really wouldn't be a lot of differences. Again, essentially just a filter. > > Are there other things to consider? Does this require some dialog at > LSFMM? As raised in my reply to Daniel, I'll be at LSF/MM and happy to discuss. I'm also not a SHMEM expert, so I'm hoping at some point we'd get feedback from Hugh. -- Cheers, David / dhildenb