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[79.242.61.201]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-43ec34a7615sm53765855e9.9.2025.04.04.13.01.56 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 04 Apr 2025 13:01:57 -0700 (PDT) Message-ID: Date: Fri, 4 Apr 2025 22:01:55 +0200 Precedence: bulk X-Mailing-List: linux-doc@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v3] docs: update THP admin guide about non-tmpfs filesystem support To: Daniel Gomez Cc: Luis Chamberlain , "Pankaj Raghav (Samsung)" , Andrew Morton , Jonathan Corbet , linux-kernel@vger.kernel.org, willy@infradead.org, linux-mm@kvack.org, Bagas Sanjaya , gost.dev@samsung.com, linux-doc@vger.kernel.org, Pankaj Raghav References: <20250404140657.29285-1-kernel@pankajraghav.com> <09c13770-4d62-430a-827d-6ad35411d18c@redhat.com> <427f683b-ac68-4820-b264-4016b34df592@redhat.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: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 04.04.25 21:44, Daniel Gomez wrote: > On Fri, Apr 04, 2025 at 09:07:23PM +0100, David Hildenbrand wrote: >> On 04.04.25 19:58, Luis Chamberlain wrote: >>> On Fri, Apr 04, 2025 at 06:18:12PM +0200, David Hildenbrand wrote: >>>> On 04.04.25 17:32, Luis Chamberlain wrote: >>>>> On Fri, Apr 04, 2025 at 04:06:57PM +0200, Pankaj Raghav (Samsung) wrote: >>>>>> From: Pankaj Raghav >>>>>> >>>>>> THP support for non-tmpfs filesystem has been around for some time now. >>>>>> Update the admin guide to reflect it. >>>>>> >>>>>> While we are at it, move FilePmdMapped to previous paragraph for clarity, >>>>>> and clarify ShmemPmdMapped & ShmemHugePage. >>>>>> >>>>>> Signed-off-by: Pankaj Raghav >>>>>> Acked-by: David Hildenbrand >>>>>> --- >>>>>> >>>>>> Changes since v2: >>>>>> - Address comment from Bagas Sanjaya >>>>>> - Squash commits and Ack from David >>>>>> >>>>>> Documentation/admin-guide/mm/transhuge.rst | 22 +++++++++++++++------- >>>>>> 1 file changed, 15 insertions(+), 7 deletions(-) >>>>>> >>>>>> diff --git a/Documentation/admin-guide/mm/transhuge.rst b/Documentation/admin-guide/mm/transhuge.rst >>>>>> index dff8d5985f0f..f8aae64e38d0 100644 >>>>>> --- a/Documentation/admin-guide/mm/transhuge.rst >>>>>> +++ b/Documentation/admin-guide/mm/transhuge.rst >>>>>> @@ -12,8 +12,8 @@ using huge pages for the backing of virtual memory with huge pages >>>>>> that supports the automatic promotion and demotion of page sizes and >>>>>> without the shortcomings of hugetlbfs. >>>>>> -Currently THP only works for anonymous memory mappings and tmpfs/shmem. >>>>>> -But in the future it can expand to other filesystems. >>>>>> +Currently, THP only works for anonymous memory mappings, tmpfs/shmem and >>>>>> +filesystems that support large folios. >>>>> >>>>> That seems to allude that THP can be supported on filesystems >>>>> that suppor large folios. I don't think we want to call that THP >>>>> and that can confuse folks. Leaving "currently" also seems to >>>>> indicate that there is more work to be done for THP for filesystems >>>>> but that's not true as well. So how about something like: >>>>> >>>>> THP only works for anonymous memory mappings, and the tmpfs/shmem is the only >>>>> filesystem to support it. The alternative to THP for other filesystems is to >>>>> support large folios and with it you can end up using huge pages >>>> >>>> That makes things more complicated without a good reason. >>>> >>>> See CONFIG_READ_ONLY_THP_FOR_FS as an early usage of the term "THP" for >>>> stuff we have in the pagecache. >>> >>> OK. >>> >>>> (with large folios we now properly implement >>>> this concept, and support more than only PMD size) >>> >>> Do we really want to call large folio support on filesystems THP? >> >> Good question. >> >> "folio" is just the metadata we currently use to manage a chunk of memory, >> and a "large folio" is one that spans more than a single page -- huge page, >> large page, super page, ... in the past the metadata for that used to be a >> complicated piece of "compound page". In the future, we might call it >> differently (struct file_mem ?), who knows. >> >> So "large folio" support in a fs allows for the usage of these larger chunks >> of memory (huge pages). > > I'm a bit confused here. I thought the term 'huge pages' referred to specific > page table level like PMD, PUD, or PGD (and not PTE). And "large folio" term can > span up to anything (currently up to PMD). Could you clarify if I'm > misunderstanding something? The transhuge.rst is still a bit inconsistent and confusing, but we started generalizing the concept to special-case "Traditional THPs" to be PMD-sized THPs. See the description about mTHP in the document. If a small folio represents a single page, then a large folio represents multiple pages (large/huge/super page). But again, "folio" is the metadata we use to manage this chunk of memory. Just like we didn't call a "huge page" a "compound page" back in the days; the "compound page" was a way to manage the metadata assigned to a "larger chunk of memory covering multiple base pages" as one unit. I once had a longer writeup about all that (including how hugetlb supports different huge page sizes, how freebsd calls these thing super pages etc). This topic seems to pop every 6 months, likely we really should do a better job at documenting all that. -- Cheers, David / dhildenb