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=-5.2 required=3.0 tests=BAYES_00, 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 A35EAC433E6 for ; Fri, 28 Aug 2020 17:08:21 +0000 (UTC) Received: from lists.ozlabs.org (lists.ozlabs.org [203.11.71.2]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPS id 4A2CB20776 for ; Fri, 28 Aug 2020 17:08:21 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 4A2CB20776 Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=arm.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@lists.ozlabs.org Received: from bilbo.ozlabs.org (lists.ozlabs.org [IPv6:2401:3900:2:1::3]) by lists.ozlabs.org (Postfix) with ESMTP id 4BdQz73Df2zDqcN for ; Sat, 29 Aug 2020 03:08:19 +1000 (AEST) Authentication-Results: lists.ozlabs.org; spf=pass (sender SPF authorized) smtp.mailfrom=kernel.org (client-ip=198.145.29.99; helo=mail.kernel.org; envelope-from=cmarinas@kernel.org; receiver=) Authentication-Results: lists.ozlabs.org; dmarc=none (p=none dis=none) header.from=arm.com Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by lists.ozlabs.org (Postfix) with ESMTPS id 4BdQwn2nYPzDqdd for ; Sat, 29 Aug 2020 03:06:17 +1000 (AEST) Received: from gaia (unknown [46.69.195.127]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id 67CC620776; Fri, 28 Aug 2020 17:06:11 +0000 (UTC) Date: Fri, 28 Aug 2020 18:06:08 +0100 From: Catalin Marinas To: Will Deacon Subject: Re: Flushing transparent hugepages Message-ID: <20200828170608.GJ3169@gaia> References: <20200818150736.GQ17456@casper.infradead.org> <20200818160815.GA16191@willie-the-truck> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200818160815.GA16191@willie-the-truck> User-Agent: Mutt/1.10.1 (2018-07-13) 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, Thomas Bogendoerfer , Vineet Gupta , Russell King , Matthew Wilcox , linux-mips@vger.kernel.org, linux-mm@kvack.org, Paul Mackerras , sparclinux@vger.kernel.org, linux-snps-arc@lists.infradead.org, linuxppc-dev@lists.ozlabs.org, "David S. Miller" , linux-arm-kernel@lists.infradead.org Errors-To: linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@lists.ozlabs.org Sender: "Linuxppc-dev" On Tue, Aug 18, 2020 at 05:08:16PM +0100, Will Deacon wrote: > On Tue, Aug 18, 2020 at 04:07:36PM +0100, Matthew Wilcox wrote: > > For example, arm64 seems confused in this scenario: > > > > void flush_dcache_page(struct page *page) > > { > > if (test_bit(PG_dcache_clean, &page->flags)) > > clear_bit(PG_dcache_clean, &page->flags); > > } > > > > ... > > > > void __sync_icache_dcache(pte_t pte) > > { > > struct page *page = pte_page(pte); > > > > if (!test_and_set_bit(PG_dcache_clean, &page->flags)) > > sync_icache_aliases(page_address(page), page_size(page)); > > } > > > > So arm64 keeps track on a per-page basis which ones have been flushed. > > page_size() will return PAGE_SIZE if called on a tail page or regular > > page, but will return PAGE_SIZE << compound_order if called on a head > > page. So this will either over-flush, or it's missing the opportunity > > to clear the bits on all the subpages which have now been flushed. > > Hmm, that seems to go all the way back to 2014 as the result of a bug fix > in 923b8f5044da ("arm64: mm: Make icache synchronisation logic huge page > aware") which has a Reported-by Mark and a CC stable, suggesting something > _was_ going wrong at the time :/ Was there a point where the tail pages > could end up with PG_arch_1 uncleared on allocation? In my experience, it's the other way around: you can end up with PG_arch_1 cleared in a tail page when the head one was set (splitting THP). > > What would you _like_ to see? Would you rather flush_dcache_page() > > were called once for each subpage, or would you rather maintain > > the page-needs-flushing state once per compound page? We could also > > introduce flush_dcache_thp() if some architectures would prefer it one > > way and one the other, although that brings into question what to do > > for hugetlbfs pages. > > For arm64, we'd like to see PG_arch_1 preserved during huge page splitting > [1], but there was a worry that it might break x86 and s390. It's also not > clear to me that we can change __sync_icache_dcache() as it's called when > we're installing the entry in the page-table, so why would it be called > again for the tail pages? Indeed, __sync_icache_dcache() is called from set_pte_at() on the head page, though it could always iterate and flush the tail pages individually (I think we could have done this in commit 923b8f5044da). Currently I suspect it does some over-flushing if you use THP on executable pages (it's a no-op on non-exec pages). With MTE (arm64 memory tagging) I'm introducing a PG_arch_2 flag and losing this is more problematic as it can lead to clearing valid tags. In the subsequent patch [2], mte_sync_tags() (also called from set_pte_at()) checks the PG_arch_2 in each page of a compound one. My preference would be to treat both PG_arch_1 and _2 similarly. > [1] https://lore.kernel.org/linux-arch/20200703153718.16973-8-catalin.marinas@arm.com/ [2] https://lore.kernel.org/linux-arch/20200703153718.16973-9-catalin.marinas@arm.com/ -- Catalin