From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 1F6823B27CA for ; Fri, 17 Apr 2026 09:57:59 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1776419880; cv=none; b=InjoMtjXWkA4sfNshzIjtWJBnZaficXTlV6bQd8/R/1yrFt4wAFPzlbt1ZVxEVRINn4OFkMdwrTvMqsjB8yCXISLu+9APlfsk7brZEb5lHZFvQM/wkygKohC8E4hwBy1dab52tR4KLCUZ2+QiS7FLEFtET6QIYzSX9YpWYbnVkc= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1776419880; c=relaxed/simple; bh=2E5lhUu078wPwmjVrWs1doXuvxv4JvNpcxkCylouaek=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=sbHOfvU7laBHToQB14kC7eTexa2/4hS4W3cNJg3kxNHPCj44cc3YsEmfD6tCOyw3IofEcq+s8E5L5asZYxzB0LsXjOJPcC2p4NDSroUgj2IErR+QR9qCWi/jbGmxMZYpZHPTiLlVU92sm6JN73G/bjDxlR3GiolCuYxHNIqoftI= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=mcRcwE+C; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="mcRcwE+C" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 1FB2FC19425; Fri, 17 Apr 2026 09:57:55 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1776419879; bh=2E5lhUu078wPwmjVrWs1doXuvxv4JvNpcxkCylouaek=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; 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 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 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 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