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b=mcRcwE+CDJGe/aiLEO5IT80LYSEA2IjbnkGF+eOe7J/IAKQ00gMrplaNiY8AXfveK AtZVmL/cTmR9bB0iNBJvhU/3s6Th/qbqSWDWN22LJ5saZlDOMVVdUlW3LbaKtkrbHr LyDAqavHXgM4s7hDXNqaBUE5znLZn+0f5Bo75HcsQB6W4c/nfTOffVA9krjWSbNwtC QwROI8t6bNURjQsYgPepNr4xbipF+ca/LikVZhraBhUTFi14RZuRHuQXPVjIH0a9zC 9vFsItX6cawo42YY2M3r0USEgUa84GMSf/d/VLyXIWBkjBxBdalWUmh4BzDTVP3FKf DY27ct364FT/w== Message-ID: <8a0b6d67-121a-4ac8-a74b-737d52d8b7ac@kernel.org> Date: Fri, 17 Apr 2026 11:57:54 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v3 02/10] mm: introduce pgtable_has_pmd_leaves() To: Lance Yang , luizcap@redhat.com Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org, baolin.wang@linux.alibaba.com, ryan.roberts@arm.com, akpm@linux-foundation.org, ljs@kernel.org, ziy@nvidia.com, Liam.Howlett@oracle.com, npache@redhat.com, dev.jain@arm.com, baohua@kernel.org References: <20260410081938.51302-1-lance.yang@linux.dev> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit X-Stat-Signature: udsi5airyrwo3qbygg1hmb6sfny8dbgn X-Rspamd-Server: rspam04 X-Rspamd-Queue-Id: F1AE9140002 X-Rspam-User: X-HE-Tag: 1776419880-283754 X-HE-Meta: U2FsdGVkX1+lMd5cOWrqklcQPjE075HIfF9DqARJI08x0cb2xPpY6aRF3JEFRn13JC3ooYKTYyL/rfTTJIc+OWFvomqCCfcTYdPY8F/VtaGudWHRoqDO8tFpZjZlFxd8rQcwYyTed+PAqc/iSCTIWlRf92nAFQht4Av2g7cqiaarwVGgqoM4pmVRhcrDAbr7Gy1UFQixG6P38TzEi9z54CXji2QpOX+zqTNjl/KslakL9VMbC4bvmZtZMAP6VFCpQuoR33IA9zA9T3PKoQbYVgSj1exhHHoouNW3ZfKZZpzCeUQENDNfQyuyiYN1Ag02LcXZqMrjFv7A5nKK1sxOms9b4+kQMS4VW2tRrZ9siFDom8b5xeRNY6FfpsTMWrNAIRrZqwQlRnjdiHhNl7VBOjlPa543h00ZTodMuahZV/oPkeRr9ugYMkB9RiHmxcHPK4Ag4r0Hv9ShoNpYVcZwv+oag2Fayiv3cYUcKGs7eG2x7x7Kah1ZlwqF7Wjk3nGfprhZ4x6d8i8fRbWI9NPO/FA5mK8SHo61yIkDdzQKjMZCYOTMQCrfczh7aLGjuG0QHKiVmFhpfR0srlG/1LtrzUMJa0o7DEgw3eCYo9SUbaB/fco3tjQOxozrtXyxtLqSlqfAfa1Hjl0hhtzHjLoWR948Xm+Zi6SicViYPVZyfWNRJgf7tTVEkqe/kQhQGT1iP+1DVnbZAQlZxEiGfDHQE05TO+4eCoTmCkjiIsQyqY6D3cp1tvXMcDLhcvnsdKwx2Z4Zy7m+KJAXk4naQu2yJGxZ5tqXyc+DFy9qk2uMJTtSwhNeQJbtaHRrXciHefCRQYQKfS9tU2Xq8uGfRr9kLSbtDh9TU/VuHIvO+675vVsGlQ/j8wlWFBsHpeP/NCyWPW0VlFXi2KDEa/flOPLbCXWL+OY3wEMkHoj24+Jt2ysVYUTJ7GVdXZUzHpN7eoZ/t14J9cFjADi4GbS20EG ciV4t2rq v+NQuYjOxZ5A0ApMeqzIZQKTAhJc22f2UUrqu8Q/JbElCm9cOEL/+8sSXMrdbikx9mL9zPJKVHZaUeYxoSS1s51SA3I6x7CjPxMzU1iVPDg811AuaJpnRDFRPYoeHZUngs4glAXI4mMRl/BQiyIimhOcx5XSz5XtAG133m6ScY4U6q2quY1R5dMhliPsQvOBGX70FU5Evxj7OurxIo3Tw6t61ubRcuMQ5W+ABPTxBkHgS+PTdRozI8SejR0n2Yr9gkY6OFJKuADcdASFc07bfOHcCd8TU62AUCYCBuMAUAF7xE4A56xtMEXzBvq6Z7WG7Cdgmed0vC4geYQU= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 4/10/26 10:19, Lance Yang wrote: > > On Wed, Apr 08, 2026 at 04:22:57PM -0400, Luiz Capitulino wrote: >> Currently, we have two helpers that check for PMD-sized pages but have >> different names and slightly different semantics: >> >> - has_transparent_hugepage(): the name suggests it checks if THP is >> enabled, but when CONFIG_TRANSPARENT_HUGEPAGE=y and the architecture >> implements this helper, it actually checks if the CPU supports >> PMD-sized pages >> >> - thp_disabled_by_hw(): the name suggests it checks if THP is disabled >> by the hardware, but it just returns a cached value acquired with >> has_transparent_hugepage(). This helper is used in fast paths >> >> This commit introduces a new helper called pgtable_has_pmd_leaves() >> which is intended to replace both has_transparent_hugepage() and >> thp_disabled_by_hw(). pgtable_has_pmd_leaves() has very clear semantics: >> it returns true if the CPU supports PMD-sized pages and false otherwise. >> It always returns a cached value, so it can be used in fast paths. >> >> The new helper requires an initialization step which is performed by >> init_arch_has_pmd_leaves(). We call init_arch_has_pmd_leaves() early >> during boot in start_kernel() right after parse_early_param() but before >> parse_args(). This allows early_param() handlers to change CPU flags if >> needed (eg. parse_memopt() in x86-32) while also allowing users to use >> the API from __setup() handlers. >> >> The next commits will convert users of both has_transparent_hugepage() >> and thp_disabled_by_hw() to pgtable_has_pmd_leaves(). >> >> Signed-off-by: Luiz Capitulino >> --- >> include/linux/pgtable.h | 15 +++++++++++++++ >> init/main.c | 1 + >> mm/memory.c | 8 ++++++++ >> 3 files changed, 24 insertions(+) >> >> diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h >> index a50df42a893f..c4c5282f795c 100644 >> --- a/include/linux/pgtable.h >> +++ b/include/linux/pgtable.h >> @@ -2192,6 +2192,21 @@ static inline const char *pgtable_level_to_str(enum pgtable_level level) >> } >> } >> >> +#ifdef CONFIG_MMU >> +extern bool __arch_has_pmd_leaves; >> +static inline bool pgtable_has_pmd_leaves(void) >> +{ >> + return __arch_has_pmd_leaves; >> +} >> +void __init init_arch_has_pmd_leaves(void); >> +#else >> +static inline bool pgtable_has_pmd_leaves(void) >> +{ >> + return false; >> +} >> +static inline void __init init_arch_has_pmd_leaves(void) { } >> +#endif >> + >> #endif /* !__ASSEMBLY__ */ >> >> #if !defined(MAX_POSSIBLE_PHYSMEM_BITS) && !defined(CONFIG_64BIT) >> diff --git a/init/main.c b/init/main.c >> index 1cb395dd94e4..07f2ddbf9677 100644 >> --- a/init/main.c >> +++ b/init/main.c >> @@ -1044,6 +1044,7 @@ void start_kernel(void) >> print_kernel_cmdline(saved_command_line); >> /* parameters may set static keys */ >> parse_early_param(); >> + init_arch_has_pmd_leaves(); > > One more thought here: I don't see why we need boot-time caching. > > has_transparent_hugepage() does *not* look expensive on the common > archs. On x86, it is just a CPU feature check. MIPS is different, yes, > only the first call there is more involved ... > > But if we *really* want caching, couldn't we just do it lazily instead > of adding another early boot init step? > > Something like: > > bool pgtable_has_pmd_leaves(void) > { > static int __arch_has_pmd_leaves = -1; > > if (READ_ONCE(__arch_has_pmd_leaves) < 0) > WRITE_ONCE(__arch_has_pmd_leaves, has_transparent_hugepage()); > > return READ_ONCE(__arch_has_pmd_leaves); Would look easier when just using two booleans: "has_pmd_leaves" and "probed". I do like static keys, though. And wonder whether just putting that into hugepage_init() would be sufficient? -- Cheers, David