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]) by smtp.lore.kernel.org (Postfix) with ESMTP id 857CBC433EF for ; Wed, 8 Dec 2021 23:35:52 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id AD36C6B0071; Wed, 8 Dec 2021 18:35:41 -0500 (EST) Received: by kanga.kvack.org (Postfix, from userid 40) id A5B206B0073; Wed, 8 Dec 2021 18:35:41 -0500 (EST) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 8D4056B0074; Wed, 8 Dec 2021 18:35:41 -0500 (EST) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0136.hostedemail.com [216.40.44.136]) by kanga.kvack.org (Postfix) with ESMTP id 77C056B0071 for ; Wed, 8 Dec 2021 18:35:41 -0500 (EST) Received: from smtpin09.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay01.hostedemail.com (Postfix) with ESMTP id 3A9F2181951DB for ; Wed, 8 Dec 2021 23:35:31 +0000 (UTC) X-FDA: 78896236062.09.9C47616 Received: from sin.source.kernel.org (sin.source.kernel.org [145.40.73.55]) by imf03.hostedemail.com (Postfix) with ESMTP id D6CA920003 for ; Wed, 8 Dec 2021 23:35:29 +0000 (UTC) Received: from mail.kernel.org (unknown [198.145.29.99]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by sin.source.kernel.org (Postfix) with ESMTPS id EECFCCE22BC; Wed, 8 Dec 2021 23:35:26 +0000 (UTC) Received: by mail.kernel.org (Postfix) with ESMTPSA id 489AA6052B; Wed, 8 Dec 2021 23:35:25 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=linux-foundation.org; s=korg; t=1639006525; bh=M/FFLIUfopmQa9VGaPR3HduOvbiObmVnD2IIBNZzwQ0=; h=Date:From:To:Cc:Subject:In-Reply-To:References:From; b=NZ+jQH8wuhSJmUdQW/d3+4YpQGqsg9DVIMqAtFkIu+7cSFnlu/xFCeh34kMDy4UgJ algdoERSUM1AHSvxoCu+/4jpuhOFpaTthLi4eltRYVKGyGk3sg6ytahmz08JT/721o BTFEFQzyWuwbCjX6haTiT2LOKXTm4CT1eFFrFZ/U= Date: Wed, 8 Dec 2021 15:35:23 -0800 From: Andrew Morton To: Li Xinhai Cc: linux-mm@kvack.org Subject: Re: [PATCH] mm/gup.c: stricter check on THP migration entry during follow_pmd_mask Message-Id: <20211208153523.bc1810ff2049fae53b4749c2@linux-foundation.org> In-Reply-To: <20211208133930.599253-1-lixinhai.lxh@gmail.com> References: <20211208133930.599253-1-lixinhai.lxh@gmail.com> X-Mailer: Sylpheed 3.5.1 (GTK+ 2.24.31; x86_64-pc-linux-gnu) Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit X-Rspamd-Server: rspam02 X-Rspamd-Queue-Id: D6CA920003 X-Stat-Signature: ffzbanxd98jr11f81jrw9sii6i16ihps Authentication-Results: imf03.hostedemail.com; dkim=pass header.d=linux-foundation.org header.s=korg header.b=NZ+jQH8w; spf=pass (imf03.hostedemail.com: domain of akpm@linux-foundation.org designates 145.40.73.55 as permitted sender) smtp.mailfrom=akpm@linux-foundation.org; dmarc=none X-HE-Tag: 1639006529-960750 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Wed, 8 Dec 2021 21:39:30 +0800 Li Xinhai wrote: > When BUG_ON check for THP migration entry, we need also consider > !thp_migration_supported() case, becaouse it is invalid at the code > context. Now, make this check been covered without adding extra cost. > > Because pmdval instead of *pmd is read, so just call > pmd_migration_entry_wait(). > > Signed-off-by: Li Xinhai > --- > mm/gup.c | 5 ++--- > 1 file changed, 2 insertions(+), 3 deletions(-) > > diff --git a/mm/gup.c b/mm/gup.c > index 2c51e9748a6a..49f490bae030 100644 > --- a/mm/gup.c > +++ b/mm/gup.c > @@ -644,10 +644,9 @@ static struct page *follow_pmd_mask(struct vm_area_struct *vma, > if (!pmd_present(pmdval)) { > if (likely(!(flags & FOLL_MIGRATION))) > return no_page_table(vma, flags); > - VM_BUG_ON(thp_migration_supported() && > + VM_BUG_ON(!thp_migration_supported() || I don't get this. thp_migration_supported() evaluates to a compile-time constant. What is the point in doing a runtime BUG based on a thing which is known at compile-time? > !is_pmd_migration_entry(pmdval)); > - if (is_pmd_migration_entry(pmdval)) > - pmd_migration_entry_wait(mm, pmd); > + pmd_migration_entry_wait(mm, pmd); > pmdval = READ_ONCE(*pmd); > /* > * MADV_DONTNEED may convert the pmd to null because