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=-2.3 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=no 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 9CCF8C32751 for ; Wed, 31 Jul 2019 15:21:26 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 7CB36206A2 for ; Wed, 31 Jul 2019 15:21:26 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1729649AbfGaPTi (ORCPT ); Wed, 31 Jul 2019 11:19:38 -0400 Received: from mx1.redhat.com ([209.132.183.28]:5399 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1728244AbfGaPSq (ORCPT ); Wed, 31 Jul 2019 11:18:46 -0400 Received: from smtp.corp.redhat.com (int-mx08.intmail.prod.int.phx2.redhat.com [10.5.11.23]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 02BE2300D20F; Wed, 31 Jul 2019 15:18:46 +0000 (UTC) Received: from dhcp-27-174.brq.redhat.com (unknown [10.43.17.136]) by smtp.corp.redhat.com (Postfix) with SMTP id 275AF196EC; Wed, 31 Jul 2019 15:18:43 +0000 (UTC) Received: by dhcp-27-174.brq.redhat.com (nbSMTP-1.00) for uid 1000 oleg@redhat.com; Wed, 31 Jul 2019 17:18:45 +0200 (CEST) Date: Wed, 31 Jul 2019 17:18:43 +0200 From: Oleg Nesterov To: Song Liu Cc: lkml , "linux-mm@kvack.org" , "akpm@linux-foundation.org" , "matthew.wilcox@oracle.com" , "kirill.shutemov@linux.intel.com" , Kernel Team , "william.kucharski@oracle.com" , "srikar@linux.vnet.ibm.com" Subject: Re: [PATCH v10 3/4] mm, thp: introduce FOLL_SPLIT_PMD Message-ID: <20190731151842.GB25078@redhat.com> References: <20190730052305.3672336-1-songliubraving@fb.com> <20190730052305.3672336-4-songliubraving@fb.com> <20190730161113.GC18501@redhat.com> <1E2B5653-BA85-4A05-9B41-57CF9E48F14A@fb.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1E2B5653-BA85-4A05-9B41-57CF9E48F14A@fb.com> User-Agent: Mutt/1.5.24 (2015-08-30) X-Scanned-By: MIMEDefang 2.84 on 10.5.11.23 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.40]); Wed, 31 Jul 2019 15:18:46 +0000 (UTC) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 07/30, Song Liu wrote: > > > > On Jul 30, 2019, at 9:11 AM, Oleg Nesterov wrote: > > > > So after the next patch we have a single user of FOLL_SPLIT_PMD (uprobes) > > and a single user of FOLL_SPLIT: arch/s390/mm/gmap.c:thp_split_mm(). > > > > Hmm. > > I think this is what we want. :) We? I don't ;) > FOLL_SPLIT is the fallback solution for users who cannot handle THP. and again, we have a single user: thp_split_mm(). I do not know if it can use FOLL_SPLIT_PMD or not, may be you can take a look... > With > more THP aware code, there will be fewer users of FOLL_SPLIT. Fewer than 1? Good ;) > >> @@ -399,7 +399,7 @@ static struct page *follow_pmd_mask(struct vm_area_struct *vma, > >> spin_unlock(ptl); > >> return follow_page_pte(vma, address, pmd, flags, &ctx->pgmap); > >> } > >> - if (flags & FOLL_SPLIT) { > >> + if (flags & (FOLL_SPLIT | FOLL_SPLIT_PMD)) { > >> int ret; > >> page = pmd_page(*pmd); > >> if (is_huge_zero_page(page)) { > >> @@ -408,7 +408,7 @@ static struct page *follow_pmd_mask(struct vm_area_struct *vma, > >> split_huge_pmd(vma, pmd, address); > >> if (pmd_trans_unstable(pmd)) > >> ret = -EBUSY; > >> - } else { > >> + } else if (flags & FOLL_SPLIT) { > >> if (unlikely(!try_get_page(page))) { > >> spin_unlock(ptl); > >> return ERR_PTR(-ENOMEM); > >> @@ -420,6 +420,10 @@ static struct page *follow_pmd_mask(struct vm_area_struct *vma, > >> put_page(page); > >> if (pmd_none(*pmd)) > >> return no_page_table(vma, flags); > >> + } else { /* flags & FOLL_SPLIT_PMD */ > >> + spin_unlock(ptl); > >> + split_huge_pmd(vma, pmd, address); > >> + ret = pte_alloc(mm, pmd); > > > > I fail to understand why this differs from the is_huge_zero_page() case above. > > split_huge_pmd() handles is_huge_zero_page() differently. In this case, we > cannot use the pmd_trans_unstable() check. Please correct me, but iiuc the problem is not that split_huge_pmd() handles is_huge_zero_page() differently, the problem is that __split_huge_pmd_locked() handles the !vma_is_anonymous(vma) differently and returns with pmd_none() = T after pmdp_huge_clear_flush_notify(). This means that pmd_trans_unstable() will fail. Now, I don't understand why do we need pmd_trans_unstable() after split_huge_pmd(huge-zero-pmd), but whatever reason we have, why can't we unify both cases? IOW, could you explain why the path below is wrong? Oleg. --- x/mm/gup.c +++ x/mm/gup.c @@ -399,14 +399,16 @@ static struct page *follow_pmd_mask(struct vm_area_struct *vma, spin_unlock(ptl); return follow_page_pte(vma, address, pmd, flags, &ctx->pgmap); } - if (flags & FOLL_SPLIT) { + if (flags & (FOLL_SPLIT | FOLL_SPLIT_PMD)) { int ret; page = pmd_page(*pmd); - if (is_huge_zero_page(page)) { + if ((flags & FOLL_SPLIT_PMD) || is_huge_zero_page(page)) { spin_unlock(ptl); - ret = 0; split_huge_pmd(vma, pmd, address); - if (pmd_trans_unstable(pmd)) + ret = 0; + if (pte_alloc(mm, pmd)) + ret = -ENOMEM; + else if (pmd_trans_unstable(pmd)) ret = -EBUSY; } else { if (unlikely(!try_get_page(page))) {