From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out-189.mta0.migadu.com (out-189.mta0.migadu.com [91.218.175.189]) (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 34AF433DEE0 for ; Fri, 10 Apr 2026 08:19:52 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=91.218.175.189 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1775809194; cv=none; b=erZufNVWKYPJwav3BkUn1n44U9+EyHYgBjYFdJcOc9hSsMdKSUk4d5rOHY4Li7/oMSAAR9FNzwMHszR6I6Cp0Og+CAxf4zAbda88vNPT66XQCcU9Jw7Sg8O/wwQoHCrKhtkyTymPNes1Y/UAqWZtE+MU2DcfWU+55ZMRuqwIVEA= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1775809194; c=relaxed/simple; bh=QLtA6gO7oUPThjSEzDfeV4rp+/dY0R9J3m2ziTWsl6g=; h=From:To:Cc:Subject:Date:Message-Id:In-Reply-To:References: MIME-Version:Content-Type; b=X0KeCS8Qhzc1NytTTDjM9xPvXq5jUG5QxRABG33j9vsJQx2uvBvShWUWy0HMNL0ocZT8VMpb+Q/eZsXOj8zrKBXAeFq4uM89TXXOByCmt10+nZENnN09URiOUMK6gLdFefcyKntWnXnar6T5+2ccw3fmYEG4RyZbBlVgrEbJp14= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev; spf=pass smtp.mailfrom=linux.dev; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b=akl9dBtH; arc=none smtp.client-ip=91.218.175.189 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=linux.dev Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b="akl9dBtH" X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1775809190; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=zoo8eGSkihm4pzVA3O7YxgtC1cJl7QU3sLX++hivSVg=; b=akl9dBtHidnFCQuw9V6gubcDZsWwzA37+wRfKcpHSp5J4giorE+9zz+ooqFWH2mlUxi2W9 Qx+36gQebk6ImLIT4CXJKspYN9poyuLbh563Bd1vGEgEjdrNH31Rcmj6YDa9qa/NvqoQ1o Xanm0raswE1rD7u11I5yNr6+OJyysc4= From: Lance Yang To: luizcap@redhat.com 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, Lance Yang Subject: Re: [PATCH v3 02/10] mm: introduce pgtable_has_pmd_leaves() Date: Fri, 10 Apr 2026 16:19:38 +0800 Message-Id: <20260410081938.51302-1-lance.yang@linux.dev> In-Reply-To: References: Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-Migadu-Flow: FLOW_OUT 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); } 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 ... Thanks, Lance