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=-10.2 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,MENTIONS_GIT_HOSTING, SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED,USER_AGENT_SANE_1 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 8C4D3C4363C for ; Mon, 21 Sep 2020 20:00:27 +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 B94012193E for ; Mon, 21 Sep 2020 20:00:26 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org B94012193E Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=intel.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 4BwFfc38D9zDqq0 for ; Tue, 22 Sep 2020 06:00:24 +1000 (AEST) Authentication-Results: lists.ozlabs.org; spf=pass (sender SPF authorized) smtp.mailfrom=intel.com (client-ip=192.55.52.151; helo=mga17.intel.com; envelope-from=ira.weiny@intel.com; receiver=) Authentication-Results: lists.ozlabs.org; dmarc=pass (p=none dis=none) header.from=intel.com Received: from mga17.intel.com (mga17.intel.com [192.55.52.151]) (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 4BwFcQ6mtLzDqnZ for ; Tue, 22 Sep 2020 05:58:29 +1000 (AEST) IronPort-SDR: VxAL+SV449/oj7KM/pE+JeN46iS6TFHH03IR5e7G5TF4FgBtRRxt7CGCazh6oS1UurXCnqt+H/ 15voUTUcyQhA== X-IronPort-AV: E=McAfee;i="6000,8403,9751"; a="140471501" X-IronPort-AV: E=Sophos;i="5.77,287,1596524400"; d="scan'208";a="140471501" X-Amp-Result: SKIPPED(no attachment in message) X-Amp-File-Uploaded: False Received: from orsmga005.jf.intel.com ([10.7.209.41]) by fmsmga107.fm.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 21 Sep 2020 12:58:26 -0700 IronPort-SDR: uOMQ77kzm8Nf+yWek1kTuScXulU6TmYu78CjdACPceLW7H1QaWd6t0P9Uf87NBJvd3fOhvq6oe f2Bh5k5J2ZCg== X-IronPort-AV: E=Sophos;i="5.77,287,1596524400"; d="scan'208";a="485647918" Received: from iweiny-desk2.sc.intel.com (HELO localhost) ([10.3.52.147]) by orsmga005-auth.jf.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 21 Sep 2020 12:58:25 -0700 Date: Mon, 21 Sep 2020 12:58:25 -0700 From: Ira Weiny To: Matthew Wilcox Subject: Re: [patch RFC 00/15] mm/highmem: Provide a preemptible variant of kmap_atomic & friends Message-ID: <20200921195824.GM2540965@iweiny-DESK2.sc.intel.com> References: <20200919091751.011116649@linutronix.de> <20200919173906.GQ32101@casper.infradead.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200919173906.GQ32101@casper.infradead.org> User-Agent: Mutt/1.11.1 (2018-12-01) 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: Juri Lelli , Peter Zijlstra , Sebastian Andrzej Siewior , Joonas Lahtinen , dri-devel , linux-mips@vger.kernel.org, Ben Segall , Max Filippov , Guo Ren , linux-sparc , Vincent Chen , Will Deacon , Ard Biesheuvel , linux-arch , Vincent Guittot , Herbert Xu , the arch/x86 maintainers , Russell King , linux-csky@vger.kernel.org, David Airlie , Mel Gorman , "open list:SYNOPSYS ARC ARCHITECTURE" , linux-xtensa@linux-xtensa.org, Paul McKenney , intel-gfx , linuxppc-dev , Steven Rostedt , Jani Nikula , Rodrigo Vivi , Thomas Gleixner , Dietmar Eggemann , Linux ARM , Chris Zankel , Michal Simek , Thomas Bogendoerfer , Nick Hu , Linux-MM , Linus Torvalds , LKML , Arnd Bergmann , Daniel Vetter , Vineet Gupta , Paul Mackerras , Andrew Morton , Daniel Bristot de Oliveira , "David S. Miller" , Greentime Hu Errors-To: linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@lists.ozlabs.org Sender: "Linuxppc-dev" On Sat, Sep 19, 2020 at 06:39:06PM +0100, Matthew Wilcox wrote: > On Sat, Sep 19, 2020 at 10:18:54AM -0700, Linus Torvalds wrote: > > On Sat, Sep 19, 2020 at 2:50 AM Thomas Gleixner wrote: > > > > > > this provides a preemptible variant of kmap_atomic & related > > > interfaces. This is achieved by: > > > > Ack. This looks really nice, even apart from the new capability. > > > > The only thing I really reacted to is that the name doesn't make sense > > to me: "kmap_temporary()" seems a bit odd. > > > > Particularly for an interface that really is basically meant as a > > better replacement of "kmap_atomic()" (but is perhaps also a better > > replacement for "kmap()"). > > > > I think I understand how the name came about: I think the "temporary" > > is there as a distinction from the "longterm" regular kmap(). So I > > think it makes some sense from an internal implementation angle, but I > > don't think it makes a lot of sense from an interface name. > > > > I don't know what might be a better name, but if we want to emphasize > > that it's thread-private and a one-off, maybe "local" would be a > > better naming, and make it distinct from the "global" nature of the > > old kmap() interface? > > > > However, another solution might be to just use this new preemptible > > "local" kmap(), and remove the old global one entirely. Yes, the old > > global one caches the page table mapping and that sounds really > > efficient and nice. But it's actually horribly horribly bad, because > > it means that we need to use locking for them. Your new "temporary" > > implementation seems to be fundamentally better locking-wise, and only > > need preemption disabling as locking (and is equally fast for the > > non-highmem case). > > > > So I wonder if the single-page TLB flush isn't a better model, and > > whether it wouldn't be a lot simpler to just get rid of the old > > complex kmap() entirely, and replace it with this? > > > > I agree we can't replace the kmap_atomic() version, because maybe > > people depend on the preemption disabling it also implied. But what > > about replacing the non-atomic kmap()? > > My concern with that is people might use kmap() and then pass the address > to a different task. So we need to audit the current users of kmap() > and convert any that do that into using vmap() instead. > I've done some of this work.[3] PKS and pmem stray write protection[2] depend on kmap to enable the correct PKS settings. After working through the exception handling we realized that some users of kmap() seem to be doing just this; passing the address to a different task. >From what I have found ~90% of kmap() callers are 'kmap_thread()' and the other ~10% are kmap().[3] But of those 10% I'm not familiar with the code enough to know if they really require a 'global' map. What I do know is they save an address which appears to be used in other threads. But I could be wrong. For PKS I added a 'global' implementation which could then be called by kmap() and added a new kmap_thread() call which used the original 'local' version of the PKS interface. The PKS work is still being reviewed internally for the TIP core code. But I've pushed it all to git hub for purposes of this discussion.[1] > I like kmap_local(). Or kmap_thread(). I chose kmap_thread() so that makes sense to me. I also thought about using kmap_global() as an alternative interface which would change just ~10% of the callers and make the series much smaller. But internal discussions lead me to chose kmap_thread() as the new interface so that we don't change the semantics of kmap(). Ira [1] https://github.com/weiny2/linux-kernel/tree/lm-pks-pmem-for-5.10-v3 [2] https://lore.kernel.org/lkml/20200717072056.73134-1-ira.weiny@intel.com/ [3] 12:42:06 > git grep ' kmap(' *.c | grep -v '* ' | wc -l 22 12:43:32 > git grep ' kmap_thread(' *.c | grep -v '* ' | wc -l 204 Here are the callers which hand an address to another thread. 12:45:25 > git grep ' kmap(' *.c | grep -v '* ' arch/x86/mm/dump_pagetables.c: [PKMAP_BASE_NR] = { 0UL, "Persistent kmap() Area" }, drivers/firewire/net.c: ptr = kmap(dev->broadcast_rcv_buffer.pages[u]); drivers/gpu/drm/i915/gem/i915_gem_pages.c: return kmap(sg_page(sgt->sgl)); drivers/gpu/drm/i915/selftests/i915_perf.c: scratch = kmap(ce->vm->scratch[0].base.page); drivers/gpu/drm/ttm/ttm_bo_util.c: map->virtual = kmap(map->page); drivers/infiniband/hw/qib/qib_user_sdma.c: mpage = kmap(page); drivers/misc/vmw_vmci/vmci_host.c: context->notify = kmap(context->notify_page) + (uva & (PAGE_SIZE - 1)); drivers/misc/xilinx_sdfec.c: addr = kmap(pages[i]); drivers/mmc/host/usdhi6rol0.c: host->pg.mapped = kmap(host->pg.page); drivers/mmc/host/usdhi6rol0.c: host->pg.mapped = kmap(host->pg.page); drivers/mmc/host/usdhi6rol0.c: host->pg.mapped = kmap(host->pg.page); drivers/nvme/target/tcp.c: iov->iov_base = kmap(sg_page(sg)) + sg->offset + sg_offset; drivers/scsi/libiscsi_tcp.c: segment->sg_mapped = kmap(sg_page(sg)); drivers/target/iscsi/iscsi_target.c: iov[i].iov_base = kmap(sg_page(sg)) + sg->offset + page_off; drivers/target/target_core_transport.c: return kmap(sg_page(sg)) + sg->offset; fs/btrfs/check-integrity.c: block_ctx->datav[i] = kmap(block_ctx->pagev[i]); fs/ceph/dir.c: cache_ctl->dentries = kmap(cache_ctl->page); fs/ceph/inode.c: ctl->dentries = kmap(ctl->page); lib/scatterlist.c: miter->addr = kmap(miter->page) + miter->__offset; net/ceph/pagelist.c: pl->mapped_tail = kmap(page); net/ceph/pagelist.c: pl->mapped_tail = kmap(page); virt/kvm/kvm_main.c: hva = kmap(page);