From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out-178.mta0.migadu.com (out-178.mta0.migadu.com [91.218.175.178]) (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 D07183B388A for ; Mon, 13 Apr 2026 15:45:34 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=91.218.175.178 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1776095137; cv=none; b=IYKRQGTd37Zpn2+r5XdLufLVQNQM0UWtbzAVsfQprv3jLVpDrZicCWjy6NxOXJqfOSl2Qas8UFiy/x3zvUtdAOLLBezE0rTm388LzYThvqwPh0+hxaHVEt+q2WIBTHqtgwDbhVALhg36PjWNgvD1G2ZAcbR8vaj39oxWXWqMX2E= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1776095137; c=relaxed/simple; bh=NKdlCS2CnL8oiYS3G6/aX3Fl9Apm1Sl8gZLNWlmBbmQ=; h=From:To:Cc:Subject:Date:Message-Id:In-Reply-To:References: MIME-Version:Content-Type; b=pVHWxZoophk4te4gUI0odWnSwjUPRmDKq8y1xN4wPyLrbZWq3J7YFnK6e12GzBQMw6IRV9r1hC4GLW1Fn+CnQa1wO2+MsyZbxcQsWkH4RHqNjxm//sUOQzkB4xMHpLoHVL2+XmXFvTy73syiQFeaZTUV65A1VOQjlAXaxANJCJE= 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=cLPw9ob1; arc=none smtp.client-ip=91.218.175.178 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="cLPw9ob1" 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=1776095132; 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=ys8CmhQ7tTcwE6cYI4Tke5lYbAx5Yj85GkT45RjDRjA=; b=cLPw9ob1rZg8ye/toE5dQC6l55rDaUW+JG6LxMA9Khl6/hzAO1XFwO9ADSskcRb4u6of9N xd2ZPo566poANe/pzSrmGxHvs4WQyYtQtpkmZUALLbFz8bHBo+PbsWkPl0uKVrU7pXzQTZ 8X/kMHUYIJQUd1hxS7sXiFdTlrMK1aM= From: Lance Yang To: luizcap@redhat.com Cc: lance.yang@linux.dev, 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 Subject: Re: [PATCH v3 02/10] mm: introduce pgtable_has_pmd_leaves() Date: Mon, 13 Apr 2026 23:45:14 +0800 Message-Id: <20260413154514.94193-1-lance.yang@linux.dev> In-Reply-To: <119dfe3d-4cfd-46e3-a539-8ac9af90f940@redhat.com> References: <119dfe3d-4cfd-46e3-a539-8ac9af90f940@redhat.com> 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 Mon, Apr 13, 2026 at 11:24:22AM -0400, Luiz Capitulino wrote: >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. Yeah, I see that, but I'm not a big fan of carrying that over into a new generic API :) The existing implict dependency does not really look like a good reason to add another boot ordering assumption in start_kernel() if we can avoid it. I'd rather have the new helper reduce such assumption, not depend on another init point ... So if we want caching there, I still lean toward doing it lazily. But let's see what folks think :) >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); >} > >