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[2003:cb:c732:7200:c4da:f12e:1fa8:eaef]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-441b2b20a65sm138626545e9.32.2025.05.03.23.47.46 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Sat, 03 May 2025 23:47:46 -0700 (PDT) Message-ID: <9e3fb101-9a5d-43bb-924a-0df3c38333f8@redhat.com> Date: Sun, 4 May 2025 08:47:45 +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 v2 1/1] mm: fix folio_pte_batch() on XEN PV To: Andrew Morton , =?UTF-8?Q?Petr_Van=C4=9Bk?= Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, Ryan Roberts , xen-devel@lists.xenproject.org, x86@kernel.org, stable@vger.kernel.org References: <20250502215019.822-1-arkamar@atlas.cz> <20250502215019.822-2-arkamar@atlas.cz> <20250503182858.5a02729fcffd6d4723afcfc2@linux-foundation.org> 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: <20250503182858.5a02729fcffd6d4723afcfc2@linux-foundation.org> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit On 04.05.25 03:28, Andrew Morton wrote: > On Fri, 2 May 2025 23:50:19 +0200 Petr Vaněk wrote: > >> On XEN PV, folio_pte_batch() can incorrectly batch beyond the end of a >> folio due to a corner case in pte_advance_pfn(). Specifically, when the >> PFN following the folio maps to an invalidated MFN, >> >> expected_pte = pte_advance_pfn(expected_pte, nr); >> >> produces a pte_none(). If the actual next PTE in memory is also >> pte_none(), the pte_same() succeeds, >> >> if (!pte_same(pte, expected_pte)) >> break; >> >> the loop is not broken, and batching continues into unrelated memory. >> >> ... > > Looks OK for now I guess but it looks like we should pay some attention > to what types we're using. > >> --- a/mm/internal.h >> +++ b/mm/internal.h >> @@ -248,11 +248,9 @@ static inline int folio_pte_batch(struct folio *folio, unsigned long addr, >> pte_t *start_ptep, pte_t pte, int max_nr, fpb_t flags, >> bool *any_writable, bool *any_young, bool *any_dirty) >> { >> - unsigned long folio_end_pfn = folio_pfn(folio) + folio_nr_pages(folio); >> - const pte_t *end_ptep = start_ptep + max_nr; >> pte_t expected_pte, *ptep; >> bool writable, young, dirty; >> - int nr; >> + int nr, cur_nr; >> >> if (any_writable) >> *any_writable = false; >> @@ -265,11 +263,15 @@ static inline int folio_pte_batch(struct folio *folio, unsigned long addr, >> VM_WARN_ON_FOLIO(!folio_test_large(folio) || max_nr < 1, folio); >> VM_WARN_ON_FOLIO(page_folio(pfn_to_page(pte_pfn(pte))) != folio, folio); >> >> + /* Limit max_nr to the actual remaining PFNs in the folio we could batch. */ >> + max_nr = min_t(unsigned long, max_nr, >> + folio_pfn(folio) + folio_nr_pages(folio) - pte_pfn(pte)); >> + > > Methinks max_nr really wants to be unsigned long. We only batch within a single PTE table, so an integer was sufficient. The unsigned value is the result of a discussion with Ryan regarding similar/related (rmap) functions: " Personally I'd go with signed int (since that's what all the counters in struct folio that we are manipulating are, underneath the atomic_t) then check that nr_pages > 0 in __folio_rmap_sanity_checks(). " https://lore.kernel.org/linux-mm/20231204142146.91437-14-david@redhat.com/T/#ma0bfff0102f0f2391dfa94aa22a8b7219b92c957 As soon as we let "max_nr" be an "unsigned long", also the return value should be an "unsigned long", and everybody calling that function. In this case here, we should likely just use whatever type "max_nr" is. Not sure myself if we should change that here to unsigned long or long. Some callers also operate with the negative values IIRC (e.g., adjust the RSS by doing -= nr). > That will permit the > cleanup of quite a bit of truncation, extension, signedness conversion > and general type chaos in folio_pte_batch()'s various callers. > > And... > > Why does folio_nr_pages() return a signed quantity? It's a count. A partial answer is in 1ea5212aed068 ("mm: factor out large folio handling from folio_nr_pages() into folio_large_nr_pages()"), where I stumbled over the reason for a signed value myself and at least made the other functions be consistent with folio_nr_pages(): " While at it, let's consistently return a "long" value from all these similar functions. Note that we cannot use "unsigned int" (even though _folio_nr_pages is of that type), because it would break some callers that do stuff like "-folio_nr_pages()". Both "int" or "unsigned long" would work as well. " Note that folio_nr_pages() returned a "long" since the very beginning. Probably using a signed value for consistency because also mapcounts / refcounts are all signed. > > And why the heck is folio_pte_batch() inlined? It's larger then my > first hard disk and it has five callsites! :) In case of fork/zap we really want it inlined because (1) We want to optimize out all of the unnecessary checks we added for other users (2) Zap/fork code is very sensitive to function call overhead Probably, as that function sees more widespread use, we might want a non-inlined variant that can be used in places where performance doesn't matter all that much (although I am not sure there will be that many). -- Cheers, David / dhildenb