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[2003:d8:2f09:3f00:d228:bd67:7baa:d604]) by smtp.gmail.com with ESMTPSA id a640c23a62f3a-a7dc9d437b0sm854325966b.114.2024.08.09.10.25.37 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 09 Aug 2024 10:25:38 -0700 (PDT) Message-ID: Date: Fri, 9 Aug 2024 19:25:36 +0200 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 06/19] mm/pagewalk: Check pfnmap early for folio_walk_start() To: Peter Xu Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, Sean Christopherson , Oscar Salvador , Jason Gunthorpe , Axel Rasmussen , linux-arm-kernel@lists.infradead.org, x86@kernel.org, Will Deacon , Gavin Shan , Paolo Bonzini , Zi Yan , Andrew Morton , Catalin Marinas , Ingo Molnar , Alistair Popple , Borislav Petkov , Thomas Gleixner , kvm@vger.kernel.org, Dave Hansen , Alex Williamson , Yan Zhao References: <20240809160909.1023470-1-peterx@redhat.com> <20240809160909.1023470-7-peterx@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 09.08.24 18:54, Peter Xu wrote: > On Fri, Aug 09, 2024 at 06:20:06PM +0200, David Hildenbrand wrote: >> On 09.08.24 18:08, Peter Xu wrote: >>> Pfnmaps can always be identified with special bits in the ptes/pmds/puds. >>> However that's unnecessary if the vma is stable, and when it's mapped under >>> VM_PFNMAP | VM_IO. >>> >>> Instead of adding similar checks in all the levels for huge pfnmaps, let >>> folio_walk_start() fail even earlier for these mappings. It's also >>> something gup-slow already does, so make them match. >>> >>> Cc: David Hildenbrand >>> Signed-off-by: Peter Xu >>> --- >>> mm/pagewalk.c | 5 +++++ >>> 1 file changed, 5 insertions(+) >>> >>> diff --git a/mm/pagewalk.c b/mm/pagewalk.c >>> index cd79fb3b89e5..fd3965efe773 100644 >>> --- a/mm/pagewalk.c >>> +++ b/mm/pagewalk.c >>> @@ -727,6 +727,11 @@ struct folio *folio_walk_start(struct folio_walk *fw, >>> p4d_t *p4dp; >>> mmap_assert_locked(vma->vm_mm); >>> + >>> + /* It has no folio backing the mappings at all.. */ >>> + if (vma->vm_flags & (VM_IO | VM_PFNMAP)) >>> + return NULL; >>> + >> >> That is in general not what we want, and we still have some places that >> wrongly hard-code that behavior. >> >> In a MAP_PRIVATE mapping you might have anon pages that we can happily walk. >> >> vm_normal_page() / vm_normal_page_pmd() [and as commented as a TODO, >> vm_normal_page_pud()] should be able to identify PFN maps and reject them, >> no? > > Yep, I think we can also rely on special bit. > > When I was working on this whole series I must confess I am already > confused on the real users of MAP_PRIVATE pfnmaps. E.g. we probably don't > need either PFNMAP for either mprotect/fork/... at least for our use case, > then VM_PRIVATE is even one step further. Yes, it's rather a corner case indeed. > > Here I chose to follow gup-slow, and I suppose you meant that's also wrong? I assume just nobody really noticed, just like nobody noticed that walk_page_test() skips VM_PFNMAP (but not VM_IO :) ). Your process memory stats will likely miss anon folios on COW PFNMAP mappings ... in the rare cases where they exist (e.g., mmap() of /dev/mem). > If so, would it make sense we keep them aligned as of now, and change them > altogether? Or do you think we should just rely on the special bits? GUP already refuses to work on a lot of other stuff, so likely not a good use of time unless somebody complains. But yes, long-term we should make all code either respect that it could happen (and bury less awkward checks in page table walkers) or rip support for MAP_PRIVATE PFNMAP out completely. > > And, just curious: is there any use case you're aware of that can benefit > from caring PRIVATE pfnmaps yet so far, especially in this path? In general MAP_PRIVATE pfnmaps is not really useful on things like MMIO. There was a discussion (in VM_PAT) some time ago whether we could remove MAP_PRIVATE PFNMAPs completely [1]. At least some users still use COW mappings on /dev/mem, although not many (and they might not actually write to these areas). I'm happy if someone wants to try ripping that out, I'm not brave enough :) [1] https://lkml.kernel.org/r/1f2a8ed4-aaff-4be7-b3b6-63d2841a2908@redhat.com > > As far as I read, none of folio_walk_start() users so far should even > stumble on top of a pfnmap, share or private. But that's a fairly quick > glimps only. do_pages_stat()->do_pages_stat_array() should be able to trigger it, if you pass "nodes=NULL" to move_pages(). Maybe s390x could be tricked into it, but likely as you say, most code shouldn't trigger it. The function itself should be handling it correctly as of today, though. -- Cheers, David / dhildenb