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 X-Spam-Level: X-Spam-Status: No, score=-15.8 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER, INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED autolearn=unavailable autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 17B3CC433DB for ; Wed, 13 Jan 2021 10:39:59 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id C7E5B23122 for ; Wed, 13 Jan 2021 10:39:58 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727611AbhAMKj1 (ORCPT ); Wed, 13 Jan 2021 05:39:27 -0500 Received: from mx2.suse.de ([195.135.220.15]:35050 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727557AbhAMKj0 (ORCPT ); Wed, 13 Jan 2021 05:39:26 -0500 X-Virus-Scanned: by amavisd-new at test-mx.suse.de DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=suse.com; s=susede1; t=1610534319; h=from:from:reply-to:date:date:message-id:message-id:to:to:cc:cc: mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=L9WjQjeV/bLcwajRiCZA3UtPCYTm19xe+pqGl6o2F8c=; b=hSHguEU6fg658n4LF4FN7DjB0xPVe4GXgD9RYR9OxVcqKqevW2N5oT1y4wzCYVmm3KZrCU raxhsLdMKEgykt79BYTHK9QQ1OY2Sdk832B0Hq/X0Uw3mOXPMb72+fiOXtm5545TJ3kFNQ NRpq3wbh86YRYGbT+4IccWex8PGlXdM= Received: from relay2.suse.de (unknown [195.135.221.27]) by mx2.suse.de (Postfix) with ESMTP id DEF0FAC5B; Wed, 13 Jan 2021 10:38:38 +0000 (UTC) Date: Wed, 13 Jan 2021 11:38:36 +0100 From: Michal Hocko To: Muchun Song Cc: Mike Kravetz , Andrew Morton , Naoya Horiguchi , Andi Kleen , Linux Memory Management List , LKML Subject: Re: [External] Re: [PATCH v4 4/6] mm: hugetlb: retry dissolve page when hitting race Message-ID: <20210113103836.GW22493@dhcp22.suse.cz> References: <20210113052209.75531-1-songmuchun@bytedance.com> <20210113052209.75531-5-songmuchun@bytedance.com> <20210113093331.GV22493@dhcp22.suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed 13-01-21 18:14:55, Muchun Song wrote: > On Wed, Jan 13, 2021 at 5:33 PM Michal Hocko wrote: > > > > On Wed 13-01-21 13:22:07, Muchun Song wrote: > > > There is a race between dissolve_free_huge_page() and put_page(). > > > Theoretically, we should return -EBUSY when we encounter this race. > > > In fact, we have a chance to successfully dissolve the page if we > > > do a retry. Because the race window is quite small. If we seize > > > this opportunity, it is an optimization for increasing the success > > > rate of dissolving page. > > > > > > Signed-off-by: Muchun Song > > > --- > > > mm/hugetlb.c | 20 ++++++++++++++++++-- > > > 1 file changed, 18 insertions(+), 2 deletions(-) > > > > > > diff --git a/mm/hugetlb.c b/mm/hugetlb.c > > > index 4a9011e12175..898e4ea43e13 100644 > > > --- a/mm/hugetlb.c > > > +++ b/mm/hugetlb.c > > > @@ -1772,6 +1772,7 @@ int dissolve_free_huge_page(struct page *page) > > > { > > > int rc = -EBUSY; > > > > > > +retry: > > > /* Not to disrupt normal path by vainly holding hugetlb_lock */ > > > if (!PageHuge(page)) > > > return 0; > > > @@ -1793,8 +1794,23 @@ int dissolve_free_huge_page(struct page *page) > > > * We should make sure that the page is already on the free list > > > * when it is dissolved. > > > */ > > > - if (unlikely(!PageHugeFreed(head))) > > > - goto out; > > > + if (unlikely(!PageHugeFreed(head))) { > > > + spin_unlock(&hugetlb_lock); > > > + > > > + /* > > > + * Theoretically, we should return -EBUSY when we > > > + * encounter this race. In fact, we have a chance > > > + * to successfully dissolve the page if we do a > > > + * retry. Because the race window is quite small. > > > + * If we seize this opportunity, it is an optimization > > > + * for increasing the success rate of dissolving page. > > > + */ > > > + while (PageHeadHuge(head) && !PageHugeFreed(head)) { > > > + cond_resched(); > > > + cpu_relax(); > > > + } > > > + goto retry; > > > > OK, so you have done the retry here. Please fold it into the previous > > patch. Also do we need cpu_relax on top of cond_resched as well? > > Because the previous patch is a bugfix and should be backprt to the other > stable tree, right? Yes, it is a bugfix but it arguably opens another issue so the follow up patch should better be applied along with it. > I just want the fix patch to be small enough. > So I do the retry in this patch. If you do not agree with this. I > will fold this into the previous patch. > > Do you mean this? > > cpu_relax(); > cond_resched(); > cpu_relax(); No, I am questiong the use of cpu_relax. What is the point? -- Michal Hocko SUSE Labs