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[2620:137:e000::1:20]) by mx.google.com with ESMTP id a10-20020a655c8a000000b00476ca041d89si12302906pgt.790.2022.12.26.17.33.10; Mon, 26 Dec 2022 17:33:19 -0800 (PST) Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20; Authentication-Results: mx.google.com; spf=pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) smtp.mailfrom=linux-kernel-owner@vger.kernel.org; dmarc=pass (p=NONE sp=NONE dis=NONE) header.from=kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S232569AbiL0BPT convert rfc822-to-8bit (ORCPT + 66 others); Mon, 26 Dec 2022 20:15:19 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:44652 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229578AbiL0BPO (ORCPT ); Mon, 26 Dec 2022 20:15:14 -0500 Received: from mail-vs1-f46.google.com (mail-vs1-f46.google.com [209.85.217.46]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id B63492AFB for ; Mon, 26 Dec 2022 17:15:12 -0800 (PST) Received: by mail-vs1-f46.google.com with SMTP id k4so7358004vsc.4 for ; Mon, 26 Dec 2022 17:15:12 -0800 (PST) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=content-transfer-encoding:cc:to:subject:message-id:date:from :in-reply-to:references:mime-version:x-gm-message-state:from:to:cc :subject:date:message-id:reply-to; bh=faex0xXiRDvF8rchFxpPffM5E3POPyWTY89h33AM+l8=; b=3kpA/Yy9XpFl3MJ6pc9Xt4JjND5AZJA5Ae0270/10Te3wR/NCUooyTAgPBmcKmDOUl 1CE4FRiNqaG7pqMHZZZhvkt1BLBXpFv9VpP5EP7DTkiBTXORPB0rNnYfCUuG9ofZVWCb ntX+u4p4q5v6aJqdi9gZqkrzK+XhpLTeteOnm2P+Mqx5Tt+CLlXcrqNK0/5ucfTrzdjY 0FuAZBuUmQvJ39WbCDSGfkghNHG3bjcckgNay+yC7jfX98gpSVxe9HKqW5F55Ou5lroe nB4UozkWCxC9yHftr+0GGf+R0xKiz+P4pamBju+QODhVI6TpxeBbE4CxJXPk1k2rciKn azww== X-Gm-Message-State: AFqh2kpM3G923R4WfTMoQU85Mf/P24tsMweLBB45fnUvifT9Q1IarlD9 PEZcCIBimUm9bShcsEZTN7kqBIywrgRC7TGoiD0= X-Received: by 2002:a05:6102:1041:b0:3c6:2426:2210 with SMTP id h1-20020a056102104100b003c624262210mr936689vsq.86.1672103711801; Mon, 26 Dec 2022 17:15:11 -0800 (PST) MIME-Version: 1.0 References: <20221220072743.3039060-1-shiyn.lin@gmail.com> <20221220072743.3039060-5-shiyn.lin@gmail.com> In-Reply-To: From: Barry Song Date: Tue, 27 Dec 2022 14:15:00 +1300 Message-ID: Subject: Re: [PATCH v3 04/14] mm/rmap: Break COW PTE in rmap walking To: Chih-En Lin Cc: Andrew Morton , Qi Zheng , David Hildenbrand , Matthew Wilcox , Christophe Leroy , John Hubbard , Nadav Amit , linux-kernel@vger.kernel.org, linux-mm@kvack.org, Steven Rostedt , Masami Hiramatsu , Peter Zijlstra , Ingo Molnar , Arnaldo Carvalho de Melo , Mark Rutland , Alexander Shishkin , Jiri Olsa , Namhyung Kim , Yang Shi , Peter Xu , "Zach O'Keefe" , "Liam R . Howlett" , Alex Sierra , Xianting Tian , Colin Cross , Suren Baghdasaryan , Pasha Tatashin , Suleiman Souhlal , Brian Geffon , Yu Zhao , Tong Tiangen , Liu Shixin , Li kunyu , Anshuman Khandual , Vlastimil Babka , Hugh Dickins , Minchan Kim , Miaohe Lin , Gautam Menghani , Catalin Marinas , Mark Brown , Will Deacon , "Eric W . Biederman" , Thomas Gleixner , Sebastian Andrzej Siewior , Andy Lutomirski , Fenghua Yu , Barret Rhoden , Davidlohr Bueso , "Jason A . Donenfeld" , Dinglan Peng , Pedro Fonseca , Jim Huang , Huichun Feng Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 8BIT X-Spam-Status: No, score=-1.6 required=5.0 tests=BAYES_00, FREEMAIL_FORGED_FROMDOMAIN,FREEMAIL_FROM,HEADER_FROM_DIFFERENT_DOMAINS, RCVD_IN_DNSWL_NONE,RCVD_IN_MSPIKE_H2,SPF_HELO_NONE,SPF_PASS autolearn=no autolearn_force=no version=3.4.6 X-Spam-Checker-Version: SpamAssassin 3.4.6 (2021-04-09) on lindbergh.monkeyblade.net Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Dec 26, 2022 at 11:56 PM Chih-En Lin wrote: > > On Mon, Dec 26, 2022 at 10:40:49PM +1300, Barry Song wrote: > > On Tue, Dec 20, 2022 at 8:25 PM Chih-En Lin wrote: > > > > > > Some of the features (unmap, migrate, device exclusive, mkclean, etc) > > > might modify the pte entry via rmap. Add a new page vma mapped walk > > > flag, PVMW_BREAK_COW_PTE, to indicate the rmap walking to break COW PTE. > > > > > > Signed-off-by: Chih-En Lin > > > --- > > > include/linux/rmap.h | 2 ++ > > > mm/migrate.c | 3 ++- > > > mm/page_vma_mapped.c | 2 ++ > > > mm/rmap.c | 12 +++++++----- > > > mm/vmscan.c | 7 ++++++- > > > 5 files changed, 19 insertions(+), 7 deletions(-) > > > > > > diff --git a/include/linux/rmap.h b/include/linux/rmap.h > > > index bd3504d11b155..d0f07e5519736 100644 > > > --- a/include/linux/rmap.h > > > +++ b/include/linux/rmap.h > > > @@ -368,6 +368,8 @@ int make_device_exclusive_range(struct mm_struct *mm, unsigned long start, > > > #define PVMW_SYNC (1 << 0) > > > /* Look for migration entries rather than present PTEs */ > > > #define PVMW_MIGRATION (1 << 1) > > > +/* Break COW-ed PTE during walking */ > > > +#define PVMW_BREAK_COW_PTE (1 << 2) > > > > > > struct page_vma_mapped_walk { > > > unsigned long pfn; > > > diff --git a/mm/migrate.c b/mm/migrate.c > > > index dff333593a8ae..a4be7e04c9b09 100644 > > > --- a/mm/migrate.c > > > +++ b/mm/migrate.c > > > @@ -174,7 +174,8 @@ void putback_movable_pages(struct list_head *l) > > > static bool remove_migration_pte(struct folio *folio, > > > struct vm_area_struct *vma, unsigned long addr, void *old) > > > { > > > - DEFINE_FOLIO_VMA_WALK(pvmw, old, vma, addr, PVMW_SYNC | PVMW_MIGRATION); > > > + DEFINE_FOLIO_VMA_WALK(pvmw, old, vma, addr, > > > + PVMW_SYNC | PVMW_MIGRATION | PVMW_BREAK_COW_PTE); > > > > > > while (page_vma_mapped_walk(&pvmw)) { > > > rmap_t rmap_flags = RMAP_NONE; > > > diff --git a/mm/page_vma_mapped.c b/mm/page_vma_mapped.c > > > index 93e13fc17d3cb..5dfc9236dc505 100644 > > > --- a/mm/page_vma_mapped.c > > > +++ b/mm/page_vma_mapped.c > > > @@ -251,6 +251,8 @@ bool page_vma_mapped_walk(struct page_vma_mapped_walk *pvmw) > > > step_forward(pvmw, PMD_SIZE); > > > continue; > > > } > > > + if (pvmw->flags & PVMW_BREAK_COW_PTE) > > > + break_cow_pte(vma, pvmw->pmd, pvmw->address); > > > if (!map_pte(pvmw)) > > > goto next_pte; > > > this_pte: > > > diff --git a/mm/rmap.c b/mm/rmap.c > > > index 2ec925e5fa6a9..b1b7dcbd498be 100644 > > > --- a/mm/rmap.c > > > +++ b/mm/rmap.c > > > @@ -807,7 +807,8 @@ static bool folio_referenced_one(struct folio *folio, > > > struct vm_area_struct *vma, unsigned long address, void *arg) > > > { > > > struct folio_referenced_arg *pra = arg; > > > - DEFINE_FOLIO_VMA_WALK(pvmw, folio, vma, address, 0); > > > + /* it will clear the entry, so we should break COW PTE. */ > > > + DEFINE_FOLIO_VMA_WALK(pvmw, folio, vma, address, PVMW_BREAK_COW_PTE); > > > > what do you mean by breaking cow pte? in memory reclamation case, we are only > > checking and clearing page referenced bit in pte, do we really need to > > break cow? > > Since we might clear page referenced bit, it will modify the write > protection shared page table (COW-ed PTE). We should duplicate it. > > Actually, I didn’t break COW at first because it will conditionally > modify the table and only clear the referenced bit. > So, if clearing page referenced bit is fine to the COW-ed PTE table > and the break COW PTE is unnecessary here, we can remove it. if a page is mapped by 100 processes and anyone of these 100 processes access this page, we will get a reference bit in the PTE. Otherwise, we will have to scan 100 PTEs to figure out if a page is accessed and should be kept in LRU. i don't see the fundamental necessity to duplicate PTE only because of clearing the reference bit. as keeping the pte shared will help save a lot of cost for memory reclamation for those CPUs which have hardware reference bits in PTE. > > Thanks, > Chih-En Lin Thanks barry