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Tue, 09 Jul 2024 20:52:45 -0700 (PDT) Received: from [172.20.2.228] ([4.28.11.157]) by smtp.gmail.com with ESMTPSA id 98e67ed59e1d1-2c99a961f50sm10898810a91.21.2024.07.09.20.52.44 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 09 Jul 2024 20:52:44 -0700 (PDT) Message-ID: <9d5980e3-72e6-4848-b1ac-83ffab8522c4@redhat.com> Date: Wed, 10 Jul 2024 05:52:43 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 00/45] hugetlb pagewalk unification To: Oscar Salvador Cc: Peter Xu , Andrew Morton , linux-kernel@vger.kernel.org, linux-mm@kvack.org, Muchun Song , SeongJae Park , Miaohe Lin , Michal Hocko , Matthew Wilcox , Christophe Leroy , Jason Gunthorpe References: <20240704043132.28501-1-osalvador@suse.de> <617169bc-e18c-40fa-be3a-99c118a6d7fe@redhat.com> <84d4e799-90da-487e-adba-6174096283b5@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: 7bit X-Stat-Signature: dd8o3w696tm4fp4bioianjwhxfi43onk X-Rspam-User: X-Rspamd-Queue-Id: D3B62180007 X-Rspamd-Server: rspam02 X-HE-Tag: 1720583570-670869 X-HE-Meta: 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 r7yO0Q94 igrrY+u6luUu3dkVOOgyLQef7d7vXS/r360zZZ7+2N+ytQ53149sDAIM82viZyGGCt0SX0vOy48b5gAsPBMZXS1uSLh20KQUzVwcbftC6Se27KrRMnHCGLIQlrEoj4A7hQgm1ytJK87okz+3m9Szjnt1w7g6A5m7uH79q74ZiJUWUoMoXum3xdyeeJkYCjsQRwbdTrjV6LEqqj0ZXSAJyZNwKebyUoYJB9gMULrk0TJK+KUU5xh9YrD0n+geoYE6VtSQGV4rtXa5XqTL2951X3YGYE8bOHV6NWEJWAInji/zBDM3Fk6Kkar5E6KXgYZeO+JSm4rrgGH/j9yeRrp7ui8c3KGgRlggDbG1fFEPE8+Xz8QuoEdk2RXJRpzmVebK8evD9pLWpAdS1AvE= 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 08.07.24 10:18, Oscar Salvador wrote: > On Thu, Jul 04, 2024 at 05:23:30PM +0200, David Hildenbrand wrote: >> My thinking was if "remove hugetlb_entry" cannot wait for "remove >> page_walk", because we found a reasonable way to do it better and convert >> the individual users. Maybe it can't. >> >> I've not given up hope that we can end up with something better and clearer >> than the current page_walk API :) > > Hi David, > Hi! > I agree that the current page_walk might be a bit convoluted, and that the > indirect functions approach is a bit of a hassle. > Having said that, let me clarify something. > > Although this patchset touches the page_walk API wrt. getting rid of > hugetlb special casing all over the place, my goal was not as focused on > the page_walk as it was on the hugetlb code to gain hability to be > interpreted on PUD/PMD level. I understand that. And it would all be easier+more straight forward if we wouldn't have that hugetlb CONT-PTE / CONT-PMD stuff in there that works similar, but different to "ordinary" cont-pte for thp. I'm sure you stumbled over the set_huge_pte_at() on arm64 for example. If we, at one point *don't* use these hugetlb functions right now to modify hugetlb entries, we might be in trouble. That's why I think we should maybe invest our time and effort in having a new pagewalker that will just batch such things naturally, and users that can operate on that naturally. For example: a hugetlb cont-pte-mapped folio will just naturally be reported as a "fully mapped folio", just like a THP would be if mapped in a compatible way. Yes, this requires more work, but as raised in some patches here, working on individual PTEs/PMDs for hugetlb is problematic. You have to batch every operation, to essentially teach ordinary code to do what the hugetlb_* special code would have done on cont-pte/cont-pmd things. (as a side note, cont-pte/cont-pmd should primarily be a hint from arch code on how many entries we can batch, like we do in folio_pte_batch(); point is that we want to batch also on architectures where we don't have such bits, and prepare for architectures that implement various sizes of batching; IMHO, having cont-pte/cont-pmd checks in common code is likely the wrong approach. Again, folio_pte_batch() is where we tackled the problem differently from the THP perspective) > > One of the things, among other things, that helped in creating this > mess/duplication we have wrt. hugetlb code vs mm core is that hugetlb > __always__ operates on ptes, which means that we cannot rely on the mm > core to do the right thing, and we need a bunch of hugetlb-pte functions > that knows about their thing, so we lean on that. > > IMHO, that was a mistake to start with, but I was not around when it was > introduced and maybe there were good reasons to deal with that the way > it is done. > But, the thing is that my ultimate goal, is for hugetlb code to be able > to deal with PUD/PMD (pte and cont-pte is already dealt with) just like > mm core does for THP (PUD is not supported by THP, but you get me), and > that is not that difficult to do, as this patchset tries to prove. > > Of course, for hugetlb to gain the hability to operate on PUD/PMD, this > means we need to add a fairly amount of code. e.g: for operating > hugepages on PUD level, code for markers on PUD/PMD level for > uffd/poison stuff (only dealt > on pmd/pte atm AFAIK), swap functions for PUD (is_swap_pud for PUD), etc. > Basically, almost all we did for PMD-* stuff we need it for PUD as well, > and that will be around when THP gains support for PUD if it ever does > (I guess that in a few years if memory capacity keeps increasing). > > E.g: pud_to_swp_entry to detect that a swp entry is hwpoison with > is_hwpoison_entry > > Yes, it is a hassle to have more code around, but IMO, this new code > will help us in 1) move away from __always__ operate on ptes 2) ease > integrate hugetlb code into mm core. > > I will keep working on this patchset not because of pagewalk savings, > but because I think it will help us in have hugetlb more mm-core ready, > since the current pagewalk has to test that a hugetlb page can be > properly read on PUD/PMD/PTE level no matter what: uffd for hugetlb on PUD/PMD, > hwpoison entries for swp on PUD/PMD, pud invalidating, etc. > > If that gets accomplished, I think that a fair amount of code that lives > in hugetlb.c can be deleted/converted as less special casing will be needed. > > I might be wrong and maybe I will hit a brick wall, but hopefully not. I have an idea for a better page table walker API that would try batching most entries (under one PTL), and walkers can just register for the types they want. Hoping I will find some time to at least scetch the user interface soon. That doesn't mean that this should block your work, but the cont-pte/cont/pmd hugetlb stuff is really nasty to handle here, and I don't particularly like where this is going. -- Cheers, David / dhildenb