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=ham 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 8D677C4363C for ; Mon, 21 Sep 2020 19:58:29 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 48CD32223E for ; Mon, 21 Sep 2020 19:58:29 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728472AbgIUT62 (ORCPT ); Mon, 21 Sep 2020 15:58:28 -0400 Received: from mga09.intel.com ([134.134.136.24]:6567 "EHLO mga09.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727197AbgIUT61 (ORCPT ); Mon, 21 Sep 2020 15:58:27 -0400 IronPort-SDR: rGkQ8Ecn3RdphPtP6caXGYgD+FcvA6RwBTyeWuzrNdI2h8XRvK+IUIeCT3Si0BglJpbU5to+io ADKkpkVtFatQ== X-IronPort-AV: E=McAfee;i="6000,8403,9751"; a="161391856" X-IronPort-AV: E=Sophos;i="5.77,287,1596524400"; d="scan'208";a="161391856" X-Amp-Result: SKIPPED(no attachment in message) X-Amp-File-Uploaded: False Received: from orsmga005.jf.intel.com ([10.7.209.41]) by orsmga102.jf.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 Cc: Linus Torvalds , Thomas Gleixner , LKML , linux-arch , Paul McKenney , the arch/x86 maintainers , Sebastian Andrzej Siewior , Peter Zijlstra , Juri Lelli , Vincent Guittot , Dietmar Eggemann , Steven Rostedt , Ben Segall , Mel Gorman , Daniel Bristot de Oliveira , Will Deacon , Andrew Morton , Linux-MM , Russell King , Linux ARM , Chris Zankel , Max Filippov , linux-xtensa@linux-xtensa.org, Jani Nikula , Joonas Lahtinen , Rodrigo Vivi , David Airlie , Daniel Vetter , intel-gfx , dri-devel , Ard Biesheuvel , Herbert Xu , Vineet Gupta , "open list:SYNOPSYS ARC ARCHITECTURE" , Arnd Bergmann , Guo Ren , linux-csky@vger.kernel.org, Michal Simek , Thomas Bogendoerfer , linux-mips@vger.kernel.org, Nick Hu , Greentime Hu , Vincent Chen , Michael Ellerman , Benjamin Herrenschmidt , Paul Mackerras , linuxppc-dev , "David S. Miller" , linux-sparc 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) Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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);