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 lists.ozlabs.org (lists.ozlabs.org [112.213.38.117]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id 53152C47DDF for ; Wed, 31 Jan 2024 10:32:56 +0000 (UTC) Received: from boromir.ozlabs.org (localhost [IPv6:::1]) by lists.ozlabs.org (Postfix) with ESMTP id 4TPyzW0F2Gz3cXQ for ; Wed, 31 Jan 2024 21:32:55 +1100 (AEDT) Authentication-Results: lists.ozlabs.org; spf=pass (sender SPF authorized) smtp.mailfrom=arm.com (client-ip=217.140.110.172; helo=foss.arm.com; envelope-from=ryan.roberts@arm.com; receiver=lists.ozlabs.org) Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by lists.ozlabs.org (Postfix) with ESMTP id 4TPyz35Ky5z2ykn for ; Wed, 31 Jan 2024 21:32:30 +1100 (AEDT) Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id A3E8FDA7; Wed, 31 Jan 2024 02:32:42 -0800 (PST) Received: from [10.57.79.60] (unknown [10.57.79.60]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id A65A23F5A1; Wed, 31 Jan 2024 02:31:53 -0800 (PST) Message-ID: <424115a2-a924-4c28-8027-32db6ab9278d@arm.com> Date: Wed, 31 Jan 2024 10:31:51 +0000 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v1 9/9] mm/memory: optimize unmap/zap with PTE-mapped THP Content-Language: en-GB To: David Hildenbrand , linux-kernel@vger.kernel.org References: <20240129143221.263763-1-david@redhat.com> <20240129143221.263763-10-david@redhat.com> From: Ryan Roberts In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-BeenThere: linuxppc-dev@lists.ozlabs.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: linux-arch@vger.kernel.org, linux-s390@vger.kernel.org, Alexander Gordeev , Christian Borntraeger , Arnd Bergmann , Vasily Gorbik , Peter Zijlstra , "Aneesh Kumar K.V" , linuxppc-dev@lists.ozlabs.org, Matthew Wilcox , linux-mm@kvack.org, Nick Piggin , Catalin Marinas , "Naveen N. Rao" , Andrew Morton , Will Deacon , Heiko Carstens , Sven Schnelle Errors-To: linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@lists.ozlabs.org Sender: "Linuxppc-dev" On 31/01/2024 10:21, David Hildenbrand wrote: > >>> + >>> +#ifndef clear_full_ptes >>> +/** >>> + * clear_full_ptes - Clear PTEs that map consecutive pages of the same folio. >> >> I know its implied from "pages of the same folio" (and even more so for the >> above variant due to mention of access/dirty), but I wonder if its useful to >> explicitly state that "all ptes being cleared are present at the time of the >> call"? > > "Clear PTEs" -> "Clear present PTEs" ? > > That should make it clearer. Works for me. > > [...] > >>>       if (!delay_rmap) { >>> -        folio_remove_rmap_pte(folio, page, vma); >>> +        folio_remove_rmap_ptes(folio, page, nr, vma); >>> + >>> +        /* Only sanity-check the first page in a batch. */ >>>           if (unlikely(page_mapcount(page) < 0)) >>>               print_bad_pte(vma, addr, ptent, page); >> >> Is there a case for either removing this all together or moving it into >> folio_remove_rmap_ptes()? It seems odd to only check some pages. >> > > I really wanted to avoid another nasty loop here. > > In my thinking, for 4k folios, or when zapping subpages of large folios, we > still perform the exact same checks. Only when batching we don't. So if there is > some problem, there are ways to get it triggered. And these problems are barely > ever seen. > > folio_remove_rmap_ptes() feels like the better place -- especially because the > delayed-rmap handling is effectively unchecked. But in there, we cannot > "print_bad_pte()". > > [background: if we had a total mapcount -- iow cheap folio_mapcount(), I'd check > here that the total mapcount does not underflow, instead of checking per-subpage] All good points... perhaps extend the comment to describe how this could be solved in future with cheap total_mapcount()? Or in the commit log if you prefer? > >> >>>       } >>> -    if (unlikely(__tlb_remove_page(tlb, page, delay_rmap))) { >>> +    if (unlikely(__tlb_remove_folio_pages(tlb, page, nr, delay_rmap))) { >>>           *force_flush = true; >>>           *force_break = true; >>>       } >>>   } >>>   -static inline void zap_present_pte(struct mmu_gather *tlb, >>> +/* >>> + * Zap or skip one present PTE, trying to batch-process subsequent PTEs that >>> map >> >> Zap or skip *at least* one... ? > > Ack >