From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) (using TLSv1 with cipher DHE-RSA-AES256-SHA (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id 5D3DDF34C5C for ; Mon, 13 Apr 2026 15:45:38 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id CB1A86B0096; Mon, 13 Apr 2026 11:45:37 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id C3B316B0098; Mon, 13 Apr 2026 11:45:37 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id ADBDC6B0099; Mon, 13 Apr 2026 11:45:37 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from relay.hostedemail.com (smtprelay0014.hostedemail.com [216.40.44.14]) by kanga.kvack.org (Postfix) with ESMTP id 9C77B6B0096 for ; Mon, 13 Apr 2026 11:45:37 -0400 (EDT) Received: from smtpin20.hostedemail.com (a10.router.float.18 [10.200.18.1]) by unirelay04.hostedemail.com (Postfix) with ESMTP id 2B23A1A023F for ; Mon, 13 Apr 2026 15:45:37 +0000 (UTC) X-FDA: 84653957514.20.74FD6C8 Received: from out-178.mta0.migadu.com (out-178.mta0.migadu.com [91.218.175.178]) by imf03.hostedemail.com (Postfix) with ESMTP id 2CEE320013 for ; Mon, 13 Apr 2026 15:45:34 +0000 (UTC) Authentication-Results: imf03.hostedemail.com; dkim=pass header.d=linux.dev header.s=key1 header.b=cLPw9ob1; spf=pass (imf03.hostedemail.com: domain of lance.yang@linux.dev designates 91.218.175.178 as permitted sender) smtp.mailfrom=lance.yang@linux.dev; dmarc=pass (policy=none) header.from=linux.dev ARC-Authentication-Results: i=1; imf03.hostedemail.com; dkim=pass header.d=linux.dev header.s=key1 header.b=cLPw9ob1; spf=pass (imf03.hostedemail.com: domain of lance.yang@linux.dev designates 91.218.175.178 as permitted sender) smtp.mailfrom=lance.yang@linux.dev; dmarc=pass (policy=none) header.from=linux.dev ARC-Seal: i=1; s=arc-20220608; d=hostedemail.com; t=1776095135; a=rsa-sha256; cv=none; b=kJW7HGw1ovqO14q48t3weLyudU3yYHNEBwK/v9c3MRdQshSj+ji1gZwPlBFjwEigfZgFir qBQuFLjom+wY6IN3+V705iaf5zxoKjgmtFrU3i/P5UiX5Z9KAE0XwySckvNERwMq4avDfZ E5QQkcPkdkRlytJ2LhAx6fJbr9oPWVo= ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1776095135; h=from:from:sender: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:dkim-signature; bh=ys8CmhQ7tTcwE6cYI4Tke5lYbAx5Yj85GkT45RjDRjA=; b=Vy3WD9q6oXlEUPzF1NsjOMIBigExeJRG3leLyVtZz0PG9YcDzrIqt8aEVkPaSpoKeGEu2N yLaOwQEAJpvHhBe0ncC1H0V12yio7QZnpgJFzcFq48Id4M6WN2iELcxGd/FbMSnZMsI7le aobiwfm7vaLDuSifqd2OdpcZ08+wvgc= 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> MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-Migadu-Flow: FLOW_OUT X-Rspamd-Server: rspam01 X-Rspamd-Queue-Id: 2CEE320013 X-Stat-Signature: 8ba3o6xtx51zczztpawtes7mtx4xf1qw X-Rspam-User: X-HE-Tag: 1776095134-636340 X-HE-Meta: U2FsdGVkX1+lhfNmJArUp1Ahv45AtIrfJP+U7OiLAXaDCMjZUAUNsfENS2wAIs66Y8A7LcZqEAh/B2b4g1T6f6dDR+GJGbSPT2FheeQxbzP9HcSfYeh8KaHxUcq11JUtBl8lEuj0umskXZejH85GWFphdBy9+xs1T7Hw4C3xAlUGnZMeDfmDOsu4A20ZRa5B83/0Q8JT74KjsKo0Z9WHuPxysVRa+UXz/bUv588Ut0D7YHNEpjfuZ0omLILHszReSzVKGyBWDckeEllGWM9e3KRRmlKTPZZrxvjYOGsJN9xu33SvyefBNp/ouD8U4tK1vvIoqS/tLXuYHJo9WGl1Pc3QLfAM19KVm2PycYrwhyMnesqk1tapFILs8Roaf9HMFl1PPZRqvzijsov4HJxWM5CWKU5H3vaMr+We/YMRc/4KRKLxJjtdT4oh9HdV0ZU4Y7mfvfwHdmItKO3LM5A7beQgHIXckuDRyG4C3x6lcpxCAUbgQC6aWPptpPuIqDURgd/gzuCarq32C4SCIQnm4ahprVp7WxwYEe+YnREfJ9V3zxMJFDSXKxJMqC8l4OFaq5g1Gnfpcwik+fh9S5dcldasZT7FBHjuaheSUD/8xpqY0uKDTQLxfMw/MrYZCDe97+uzTpzovAHL/3DzTp/kRtZL+3eGBS/RogM469L198MShKUUQypUTe1YJRrt9IaEHyvDXqcRxkcqBpvN1UEm7EyOBznLI2onxYk1SLXMVf0mHpw7WZ7rpq2Z28wZlOokoJeyMqD7PTkhVle5IZRJxaaQxvdLvBEdM66/4tX8tc2apGMlc+XYSbMb4gfIaWKYUHok2+EJosZXlyRTpgfKTROHk4jfAhz2JQ0pMzd01IPa9PwcudZJlso1Iuora11v3Dh4w5NrqXMi/6G7W2yj2waP02AITImZxcSGZz4F6v3MWwqk1SM/YnNmT2LG+GGrK4XwzK36GHfaJ8CTjh+ 5i1fD8tb rv3ZwKfje+x2fPimQshnRyxYlvEvhWqQ6hl19eu1azvEAuZVXyv+P9z9oIptVXHhXiH2z2+OkoZ6RyJ/QN2YgEGF0xXEyogQlI/6ONWge8AZpEGC+FQ1elBPNDu8tH9kfR06ItMVoVMiubSA5dGtHGstV9S5Fw9b+nw+uS4YOzJLpDzEFjSQSVaSC9ZLNrC4DxkelODWZLx4hNi7mFuRNdcRQACHupgGjhc4ZqOeq8cIWfmTJOgFLdyzYEQ== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: 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); >} > >