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Mon, 13 Apr 2026 08:24:33 -0700 (PDT) X-Received: by 2002:a05:620a:4041:b0:8cd:bfd9:e2a2 with SMTP id af79cd13be357-8ddcfbad95amr1957593785a.43.1776093873451; Mon, 13 Apr 2026 08:24:33 -0700 (PDT) Received: from [192.168.2.110] ([69.159.169.238]) by smtp.gmail.com with ESMTPSA id af79cd13be357-8ddb9351afasm864689985a.36.2026.04.13.08.24.32 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 13 Apr 2026 08:24:33 -0700 (PDT) Message-ID: <119dfe3d-4cfd-46e3-a539-8ac9af90f940@redhat.com> Date: Mon, 13 Apr 2026 11:24:22 -0400 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 Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org, david@kernel.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> Content-Language: en-US, en-CA From: Luiz Capitulino In-Reply-To: <20260410081938.51302-1-lance.yang@linux.dev> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 2026-04-10 04:19, 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 ... The caching is not being introduced by this series. thp_disabled_by_hw() (which is used in hot paths) is checking a cached value too and this cached value is also set at boot-time by hugepage_init(). So, we keep the same behavior while extending the use of the cached value in the API (which makes sense for consistency). > 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); > } > > That would avoid depending on parse_early_param() / parse_args() > ordering, and it seems much simpler too :) > > The boot-time init step looks a bit shaky to me ... I understand that caching on first access as you suggest above is an alternative way of doing this, but having to properly order initialization is very common in start_kernel(). So, unless we identify a case where the ordering is tricky or undoable, I don't see an issue with it. Also, note that this ordering exists today but it's implicity: thp_disabled_by_hw() can only be called after hugepage_init() runs. In terms of simplicity, look at how the resulting code (using an untested version of your static key suggestion) looks in comparison to caching at first use: DEFINE_STATIC_KEY_TRUE(__arch_has_pmd_leaves_key); static inline bool pgtable_has_pmd_leaves(void) { return static_branch_likely(&__arch_has_pmd_leaves_key); } void __init init_arch_has_pmd_leaves(void) { if (!arch_has_pmd_leaves()) static_branch_disable(&__arch_has_pmd_leaves_key); }