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 74BC7C433EF for ; Wed, 19 Jan 2022 10:03:08 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id EBC986B0071; Wed, 19 Jan 2022 05:03:07 -0500 (EST) Received: by kanga.kvack.org (Postfix, from userid 40) id E6CA36B0073; Wed, 19 Jan 2022 05:03:07 -0500 (EST) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id D82BE6B0074; Wed, 19 Jan 2022 05:03:07 -0500 (EST) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0243.hostedemail.com [216.40.44.243]) by kanga.kvack.org (Postfix) with ESMTP id C85C26B0071 for ; Wed, 19 Jan 2022 05:03:07 -0500 (EST) Received: from smtpin29.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay03.hostedemail.com (Postfix) with ESMTP id 7FD8280CD16D for ; Wed, 19 Jan 2022 10:03:07 +0000 (UTC) X-FDA: 79046598414.29.2C10938 Received: from casper.infradead.org (casper.infradead.org [90.155.50.34]) by imf24.hostedemail.com (Postfix) with ESMTP id 95C4F18002E for ; Wed, 19 Jan 2022 10:03:06 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=casper.20170209; h=In-Reply-To:Content-Type:MIME-Version: References:Message-ID:Subject:Cc:To:From:Date:Sender:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description; bh=3kzXvtE5LHGh9bTwu8jZhdzSjC26IpQu1fC0KE7Jd6Q=; b=C6FU+F/ty02KfXJwEgFWxYmGW/ //VF9NWRMEPVbDJJ1Nifscyzr8xFzkqZtFJMJs+Jr7bcXNlWRawWlAg/w8g14A3JPgtoe17NQ4dqa agmt9vjXbFSIECtQbjj75t2Bl5NJB9wGWFSLcQlQOtFLeaw4u5IWHAOJJCui7t4g9O7aBtP0jcElV cAO3DpKSHFpZnLXe6FEYNIJMn+A3M/cXqRO/aoVI0WLdd11cpxlWlY2eM1jXJRhBIs8vTcL5JoW+h vfmz59JcDVb5GS4O73nCXrk2cLFQOkGk7XRHOGTVS/dZ3Xc7DAXqVv8F8WIJZN82RFn06XYWVQRRF wMGLPHjQ==; Received: from j217100.upc-j.chello.nl ([24.132.217.100] helo=noisy.programming.kicks-ass.net) by casper.infradead.org with esmtpsa (Exim 4.94.2 #2 (Red Hat Linux)) id 1nA7nw-00AWSf-JD; Wed, 19 Jan 2022 10:03:01 +0000 Received: from hirez.programming.kicks-ass.net (hirez.programming.kicks-ass.net [192.168.1.225]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (4096 bits) server-digest SHA256) (Client did not present a certificate) by noisy.programming.kicks-ass.net (Postfix) with ESMTPS id 30F943004AD; Wed, 19 Jan 2022 11:02:59 +0100 (CET) Received: by hirez.programming.kicks-ass.net (Postfix, from userid 1000) id 0FB0D211935FB; Wed, 19 Jan 2022 11:02:59 +0100 (CET) Date: Wed, 19 Jan 2022 11:02:59 +0100 From: Peter Zijlstra To: Peter Collingbourne Cc: Andrey Konovalov , Andrew Morton , linux-mm@kvack.org, linux-kernel@vger.kernel.org, Mel Gorman , stable@vger.kernel.org Subject: Re: [PATCH] mm/mmzone.c: fix page_cpupid_xchg_last() to READ_ONCE() the page flags Message-ID: References: <20220118230539.323058-1-pcc@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20220118230539.323058-1-pcc@google.com> X-Rspamd-Queue-Id: 95C4F18002E X-Stat-Signature: xatk19q87soffqk3as8czmcf8qa45qdh Authentication-Results: imf24.hostedemail.com; dkim=pass header.d=infradead.org header.s=casper.20170209 header.b="C6FU+F/t"; dmarc=none; spf=none (imf24.hostedemail.com: domain of peterz@infradead.org has no SPF policy when checking 90.155.50.34) smtp.mailfrom=peterz@infradead.org X-Rspamd-Server: rspam06 X-HE-Tag: 1642586586-329756 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 Tue, Jan 18, 2022 at 03:05:39PM -0800, Peter Collingbourne wrote: > After submitting a patch with a compare-exchange loop similar to this > one to set the KASAN tag in the page flags, Andrey Konovalov pointed > out that we should be using READ_ONCE() to read the page flags. Fix > it here. What does it actually fix? If it manages to split the read and read garbage the cmpxchg will fail and we go another round, no harm done. > Fixes: 75980e97dacc ("mm: fold page->_last_nid into page->flags where possible") As per the above argument, I don't think this rates a Fixes tag, there is no actual fix. > Signed-off-by: Peter Collingbourne > Link: https://linux-review.googlesource.com/id/I2e1f5b5b080ac9c4e0eb7f98768dba6fd7821693 That's that doing here? > Cc: stable@vger.kernel.org That's massively over-selling things. > --- > mm/mmzone.c | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/mm/mmzone.c b/mm/mmzone.c > index eb89d6e018e2..f84b84b0d3fc 100644 > --- a/mm/mmzone.c > +++ b/mm/mmzone.c > @@ -90,7 +90,7 @@ int page_cpupid_xchg_last(struct page *page, int cpupid) > int last_cpupid; > > do { > - old_flags = flags = page->flags; > + old_flags = flags = READ_ONCE(page->flags); > last_cpupid = page_cpupid_last(page); > > flags &= ~(LAST_CPUPID_MASK << LAST_CPUPID_PGSHIFT); I think that if you want to touch that code, something like the below makes more sense... diff --git a/mm/mmzone.c b/mm/mmzone.c index eb89d6e018e2..ed9f4bcdc9ee 100644 --- a/mm/mmzone.c +++ b/mm/mmzone.c @@ -89,13 +89,14 @@ int page_cpupid_xchg_last(struct page *page, int cpupid) unsigned long old_flags, flags; int last_cpupid; + old_flags = READ_ONCE(page->flags); do { - old_flags = flags = page->flags; - last_cpupid = page_cpupid_last(page); + flags = old_flags; + last_cpupid = (flags >> LAST_CPUPID_PGSHIFT) & LAST_CPUPID_MASK; flags &= ~(LAST_CPUPID_MASK << LAST_CPUPID_PGSHIFT); flags |= (cpupid & LAST_CPUPID_MASK) << LAST_CPUPID_PGSHIFT; - } while (unlikely(cmpxchg(&page->flags, old_flags, flags) != old_flags)); + } while (unlikely(!try_cmpxchg(&page->flags, &old_flags, flags))); return last_cpupid; }