From patchwork Thu Aug 3 14:32:06 2023 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: David Hildenbrand X-Patchwork-Id: 13340206 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 83096C04A6A for ; Thu, 3 Aug 2023 14:34:48 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S236807AbjHCOer (ORCPT ); Thu, 3 Aug 2023 10:34:47 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:44706 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S235257AbjHCOea (ORCPT ); Thu, 3 Aug 2023 10:34:30 -0400 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.129.124]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id EACF21BFA for ; Thu, 3 Aug 2023 07:32:33 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1691073153; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=ymf0b+2Kqr9mLIxdkVxybggSGs/fK9hNLVEJOY2Qbdo=; b=cVhpsIC01lenR7bEXrnq71EA5UZECwkQkjTO2L+f+8w3QrZe7lUH9g7v8/DxPMZdeALztF Lh7FEKTu3HIfdN6hMoUrQU6W1TCze7dhRXIkFdWbeGeBDeX3Ch8UuLcYWtaemgLK5aaN7O Yg8plLaQrzuSmEcPQm2OKCByCjIEKnI= Received: from mimecast-mx02.redhat.com (mimecast-mx02.redhat.com [66.187.233.88]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id us-mta-423-0ded6q7sOS-M6lJ3orix1Q-1; Thu, 03 Aug 2023 10:32:30 -0400 X-MC-Unique: 0ded6q7sOS-M6lJ3orix1Q-1 Received: from smtp.corp.redhat.com (int-mx04.intmail.prod.int.rdu2.redhat.com [10.11.54.4]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx02.redhat.com (Postfix) with ESMTPS id 3FD4C857FD1; Thu, 3 Aug 2023 14:32:29 +0000 (UTC) Received: from t14s.fritz.box (unknown [10.39.193.129]) by smtp.corp.redhat.com (Postfix) with ESMTP id 4653F201F11C; Thu, 3 Aug 2023 14:32:26 +0000 (UTC) From: David Hildenbrand To: linux-kernel@vger.kernel.org Cc: linux-mm@kvack.org, linux-fsdevel@vger.kernel.org, kvm@vger.kernel.org, linux-kselftest@vger.kernel.org, David Hildenbrand , Andrew Morton , Linus Torvalds , liubo , Peter Xu , Matthew Wilcox , Hugh Dickins , Jason Gunthorpe , John Hubbard , Mel Gorman , Shuah Khan , Paolo Bonzini , Mel Gorman Subject: [PATCH v3 5/7] pgtable: improve pte_protnone() comment Date: Thu, 3 Aug 2023 16:32:06 +0200 Message-ID: <20230803143208.383663-6-david@redhat.com> In-Reply-To: <20230803143208.383663-1-david@redhat.com> References: <20230803143208.383663-1-david@redhat.com> MIME-Version: 1.0 X-Scanned-By: MIMEDefang 3.1 on 10.11.54.4 Precedence: bulk List-ID: X-Mailing-List: kvm@vger.kernel.org Especially the "For PROT_NONE VMAs, the PTEs are not marked _PAGE_PROTNONE" part is wrong: doing an mprotect(PROT_NONE) will end up marking all PTEs on x86_64 as _PAGE_PROTNONE, making pte_protnone() indicate "yes". So let's improve the comment, so it's easier to grasp which semantics pte_protnone() actually has. Acked-by: Mel Gorman Signed-off-by: David Hildenbrand --- include/linux/pgtable.h | 16 ++++++++++------ 1 file changed, 10 insertions(+), 6 deletions(-) diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h index 6005b5dff0c1..222a33b9600d 100644 --- a/include/linux/pgtable.h +++ b/include/linux/pgtable.h @@ -1446,12 +1446,16 @@ static inline int pud_trans_unstable(pud_t *pud) #ifndef CONFIG_NUMA_BALANCING /* - * Technically a PTE can be PROTNONE even when not doing NUMA balancing but - * the only case the kernel cares is for NUMA balancing and is only ever set - * when the VMA is accessible. For PROT_NONE VMAs, the PTEs are not marked - * _PAGE_PROTNONE so by default, implement the helper as "always no". It - * is the responsibility of the caller to distinguish between PROT_NONE - * protections and NUMA hinting fault protections. + * In an inaccessible (PROT_NONE) VMA, pte_protnone() may indicate "yes". It is + * perfectly valid to indicate "no" in that case, which is why our default + * implementation defaults to "always no". + * + * In an accessible VMA, however, pte_protnone() reliably indicates PROT_NONE + * page protection due to NUMA hinting. NUMA hinting faults only apply in + * accessible VMAs. + * + * So, to reliably identify PROT_NONE PTEs that require a NUMA hinting fault, + * looking at the VMA accessibility is sufficient. */ static inline int pte_protnone(pte_t pte) {