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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 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 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 Content-Language: en-US Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit 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