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 vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 7BA3BC77B78 for ; Sun, 23 Apr 2023 01:14:28 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229971AbjDWBO0 (ORCPT ); Sat, 22 Apr 2023 21:14:26 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:57654 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229740AbjDWBOZ (ORCPT ); Sat, 22 Apr 2023 21:14:25 -0400 Received: from mga11.intel.com (mga11.intel.com [192.55.52.93]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 2FBA82680 for ; Sat, 22 Apr 2023 18:14:24 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=intel.com; i=@intel.com; q=dns/txt; s=Intel; t=1682212464; x=1713748464; h=from:to:cc:subject:references:date:in-reply-to: message-id:mime-version; bh=DKv+N5SRkC9Ahg97gWrAhHCh0HArqXpav95JmbNuJzc=; b=B3k9IK3hRKxr9rmW3BwuoLY3KjfvF7U05g/kdeT3jPyMbtpKCGGHJxCQ 1RpPOsgStdkFfrYl5U2G91QUP9YEaPPT+JXm4Et+mD3NGk+ZlNQjXzaCw upWiqPLM/SD1K9JipDfkKsqrI1gxpwg2Xf0+TrBs+D2P+HY6Wjuw5zytJ XpBHCRwMz0IsQU5tQ1CsLpabostTkgrsCrN+K/QP5SQxjhfpgeAFnLFNN bzKmLXdfy7iueS/UkjrZJ8Aq8530olDy7Kb/ZjAiWwwN8KEpJCyAEvjgB 41UnpRl+9LdwjgVhlWn8wGYX35pTH8IubnvgyNSLC0gokpvB50ndRCRnx Q==; X-IronPort-AV: E=McAfee;i="6600,9927,10688"; a="343707637" X-IronPort-AV: E=Sophos;i="5.99,219,1677571200"; d="scan'208";a="343707637" Received: from fmsmga001.fm.intel.com ([10.253.24.23]) by fmsmga102.fm.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 22 Apr 2023 18:14:23 -0700 X-ExtLoop1: 1 X-IronPort-AV: E=McAfee;i="6600,9927,10688"; a="836534930" X-IronPort-AV: E=Sophos;i="5.99,219,1677571200"; d="scan'208";a="836534930" Received: from yhuang6-desk2.sh.intel.com (HELO yhuang6-desk2.ccr.corp.intel.com) ([10.238.208.55]) by fmsmga001-auth.fm.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 22 Apr 2023 18:14:21 -0700 From: "Huang, Ying" To: Baolin Wang Cc: , , , , , , Subject: Re: [PATCH 2/2] mm/page_alloc: add some comments to explain the possible hole in __pageblock_pfn_to_page() References: <02defcbe9d7a797a2257e5f6a28ff7ea78e394e5.1682158312.git.baolin.wang@linux.alibaba.com> Date: Sun, 23 Apr 2023 09:13:16 +0800 In-Reply-To: <02defcbe9d7a797a2257e5f6a28ff7ea78e394e5.1682158312.git.baolin.wang@linux.alibaba.com> (Baolin Wang's message of "Sat, 22 Apr 2023 18:15:18 +0800") Message-ID: <87cz3vs8nn.fsf@yhuang6-desk2.ccr.corp.intel.com> User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/27.1 (gnu/linux) MIME-Version: 1.0 Content-Type: text/plain; charset=ascii Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Baolin Wang writes: > Now the __pageblock_pfn_to_page() is used by set_zone_contiguous(), which > checks whether the given zone contains holes, and uses pfn_to_online_page() > to validate if the start pfn is online and valid, as well as using pfn_valid() > to validate the end pfn. > > However, though the start pfn of a pageblock is valid, it can not always > guarantee the end pfn of the pageblock is also valid (may be holes) in some > cases. For example, if the pageblock order is MAX_ORDER - 1, which will fall > into 2 sub-sections, and the end pfn of the pageblock may be hole even though > the start pfn is online and valid. > > This did not break anything until now, but the zone continuous is fragile > in this possible scenario. So as previous discussion[1], it is better to > add some comments to explain this possible issue in case there are some > future pfn walkers that rely on this. > > [1] https://lore.kernel.org/all/87r0sdsmr6.fsf@yhuang6-desk2.ccr.corp.intel.com/ > > Signed-off-by: Baolin Wang > --- > mm/page_alloc.c | 8 ++++++++ > 1 file changed, 8 insertions(+) > > diff --git a/mm/page_alloc.c b/mm/page_alloc.c > index 6457b64fe562..dc4005b32ae0 100644 > --- a/mm/page_alloc.c > +++ b/mm/page_alloc.c > @@ -1502,6 +1502,14 @@ void __free_pages_core(struct page *page, unsigned int order) > * interleaving within a single pageblock. It is therefore sufficient to check > * the first and last page of a pageblock and avoid checking each individual > * page in a pageblock. > + * > + * Note: if the start pfn of a pageblock is valid, but it can not always guarantee > + * the end pfn of the pageblock is also valid (may be holes) in some cases. For "valid" sounds confusing here. pfn_valid() is true, but the pfn is considered invalid at some degree. How about the following? Note: the function may return non-NULL even if the end pfn of a pageblock is in a memory hole in some situations. For > + * example, if the pageblock order is MAX_ORDER - 1, which will fall into 2 > + * sub-sections, and the end pfn of the pageblock may be hole even though the > + * start pfn is online and valid. This did not break anything until now, but be > + * careful this possible issue when checking if the whole pfns are valid of a ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ whether all pfns of a pageblock are valid. ? > + * pageblock. > */ > struct page *__pageblock_pfn_to_page(unsigned long start_pfn, > unsigned long end_pfn, struct zone *zone) My English is poor. So, feel free to ignore the comments. Best Regards, Huang, Ying