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[2003:cb:c732:7200:c4da:f12e:1fa8:eaef]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-441b8a31695sm97450265e9.40.2025.05.04.01.58.45 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Sun, 04 May 2025 01:58:45 -0700 (PDT) Message-ID: <62990a3f-524f-4362-8f64-2fc582986eba@redhat.com> Date: Sun, 4 May 2025 10:58:44 +0200 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 Cc: =?UTF-8?Q?Petr_Van=C4=9Bk?= , 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> <9e3fb101-9a5d-43bb-924a-0df3c38333f8@redhat.com> <20250504001547.177b2aba8c2ffbfe63e0552e@linux-foundation.org> 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: <20250504001547.177b2aba8c2ffbfe63e0552e@linux-foundation.org> X-Mimecast-Spam-Score: 0 X-Mimecast-MFC-PROC-ID: Z2dqkzAgj8jVQ9U3XJf_cUyGPyPeFXFE_6HpxKPIalU_1746349127 X-Mimecast-Originator: redhat.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspamd-Server: rspam06 X-Rspamd-Queue-Id: 8AE0040003 X-Stat-Signature: c5pza7dwz8tam6jaz86pmjacgsj1uc3y X-Rspam-User: X-HE-Tag: 1746349129-491606 X-HE-Meta: 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 bjC+gCee 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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 04.05.25 09:15, Andrew Morton wrote: > On Sun, 4 May 2025 08:47:45 +0200 David Hildenbrand wrote: > >>> >>> 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). > > "rss -= nr" doesn't require, expect or anticipate that `nr' can be negative! The one thing I ran into with "unsigned int" around folio_nr_pages() was that if you pass -folio-nr_pages() into a function that expects an "long" (add vs. remove a value to a counter), then the result might not be what one would expect when briefly glimpsing at the code: #include static __attribute__((noinline)) void print(long diff) { printf("%ld\n", diff); } static int value_int() { return 12345; } static unsigned int value_unsigned_int() { return 12345; } static int value_long() { return 12345; } static unsigned long value_unsigned_long() { return 12345; } int main(void) { print(-value_int()); print(-value_unsigned_int()); print(-value_long()); print(-value_unsigned_long()); return 0; } $ ./tmp -12345 4294954951 -12345 -12345 So, I am fine with using "unsigned long" (as stated in that commit description below). > >> >>> 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. > > Geeze. > > Can we step back and look at what we're doing? Anything which counts > something (eg, has "nr" in the identifier) cannot be negative. Yes. Unless we want to catch underflows (e.g., mapcount / refcount). For "nr_pages" I agree. > > It's that damn "int" thing. I think it was always a mistake that the C > language's go-to type is a signed one. Yeah. But see above that "unsigned int" in combination with long can also cause pain. > It's a system programming > language and system software rarely deals with negative scalars. > Signed scalars are the rare case. > > I do expect that the code in and around here would be cleaner and more > reliable if we were to do a careful expunging of inappropriately signed > variables. Maybe, but it would mostly be a "int -> unsigned long" conversion, probably not much more. I'm not against cleaning that up at all. > >> >>> >>> 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). > > a quick test. > > before: > text data bss dec hex filename > 12380 470 0 12850 3232 mm/madvise.o > 52975 2689 24 55688 d988 mm/memory.o > 25305 1448 2096 28849 70b1 mm/mempolicy.o > 8573 924 4 9501 251d mm/mlock.o > 20950 5864 16 26830 68ce mm/rmap.o > > (120183) > > after: > > text data bss dec hex filename > 11916 470 0 12386 3062 mm/madvise.o > 52990 2697 24 55711 d99f mm/memory.o > 25161 1448 2096 28705 7021 mm/mempolicy.o > 8381 924 4 9309 245d mm/mlock.o > 20806 5864 16 26686 683e mm/rmap.o > > (119254) > > so uninlining saves a kilobyte of text - less than I expected but > almost 1%. As I said, for fork+zap/unmap we really want to inline -- the first two users of that function when that function was still simpler and resided in mm/memory.o. For the other users, probably okay to have a non-inlined one in mm/util.c . -- Cheers, David / dhildenb