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[209.132.183.28]) by mx.google.com with ESMTPS id z12si550335qvg.5.2019.03.08.12.06.19 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Fri, 08 Mar 2019 12:06:19 -0800 (PST) Received-SPF: pass (google.com: domain of jglisse@redhat.com designates 209.132.183.28 as permitted sender) client-ip=209.132.183.28; Authentication-Results: mx.google.com; spf=pass (google.com: domain of jglisse@redhat.com designates 209.132.183.28 as permitted sender) smtp.mailfrom=jglisse@redhat.com; dmarc=pass (p=NONE sp=NONE dis=NONE) header.from=redhat.com Received: from smtp.corp.redhat.com (int-mx01.intmail.prod.int.phx2.redhat.com [10.5.11.11]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id AA65D318A5E9; Fri, 8 Mar 2019 20:06:18 +0000 (UTC) Received: from redhat.com (ovpn-124-248.rdu2.redhat.com [10.10.124.248]) by smtp.corp.redhat.com (Postfix) with ESMTPS id F3DC8600C1; Fri, 8 Mar 2019 20:06:12 +0000 (UTC) Date: Fri, 8 Mar 2019 15:06:11 -0500 From: Jerome Glisse To: Andrea Arcangeli Cc: Jason Wang , "Michael S. Tsirkin" , kvm@vger.kernel.org, virtualization@lists.linux-foundation.org, netdev@vger.kernel.org, linux-kernel@vger.kernel.org, peterx@redhat.com, linux-mm@kvack.org Subject: Re: [RFC PATCH V2 5/5] vhost: access vq metadata through kernel virtual address Message-ID: <20190308200609.GA6969@redhat.com> References: <1551856692-3384-1-git-send-email-jasowang@redhat.com> <1551856692-3384-6-git-send-email-jasowang@redhat.com> <20190307103503-mutt-send-email-mst@kernel.org> <20190307124700-mutt-send-email-mst@kernel.org> <20190307191622.GP23850@redhat.com> <20190308194845.GC26923@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <20190308194845.GC26923@redhat.com> User-Agent: Mutt/1.10.1 (2018-07-13) X-Scanned-By: MIMEDefang 2.79 on 10.5.11.11 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.41]); Fri, 08 Mar 2019 20:06:18 +0000 (UTC) X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Fri, Mar 08, 2019 at 02:48:45PM -0500, Andrea Arcangeli wrote: > Hello Jeson, > > On Fri, Mar 08, 2019 at 04:50:36PM +0800, Jason Wang wrote: > > Just to make sure I understand here. For boosting through huge TLB, do > > you mean we can do that in the future (e.g by mapping more userspace > > pages to kenrel) or it can be done by this series (only about three 4K > > pages were vmapped per virtqueue)? > > When I answered about the advantages of mmu notifier and I mentioned > guaranteed 2m/gigapages where available, I overlooked the detail you > were using vmap instead of kmap. So with vmap you're actually doing > the opposite, it slows down the access because it will always use a 4k > TLB even if QEMU runs on THP or gigapages hugetlbfs. > > If there's just one page (or a few pages) in each vmap there's no need > of vmap, the linearity vmap provides doesn't pay off in such > case. > > So likely there's further room for improvement here that you can > achieve in the current series by just dropping vmap/vunmap. > > You can just use kmap (or kmap_atomic if you're in preemptible > section, should work from bh/irq). > > In short the mmu notifier to invalidate only sets a "struct page * > userringpage" pointer to NULL without calls to vunmap. > > In all cases immediately after gup_fast returns you can always call > put_page immediately (which explains why I'd like an option to drop > FOLL_GET from gup_fast to speed it up). By the way this is on my todo list, i want to merge HMM page snapshoting with gup code which means mostly allowing to gup_fast without taking a reference on the page (so without FOLL_GET). I hope to get to that some- time before summer. > > Then you can check the sequence_counter and inc/dec counter increased > by _start/_end. That will tell you if the page you got and you called > put_page to immediately unpin it or even to free it, cannot go away > under you until the invalidate is called. > > If sequence counters and counter tells that gup_fast raced with anyt > mmu notifier invalidate you can just repeat gup_fast. Otherwise you're > done, the page cannot go away under you, the host virtual to host > physical mapping cannot change either. And the page is not pinned > either. So you can just set the "struct page * userringpage = page" > where "page" was the one setup by gup_fast. > > When later the invalidate runs, you can just call set_page_dirty if > gup_fast was called with "write = 1" and then you clear the pointer > "userringpage = NULL". > > When you need to read/write to the memory > kmap/kmap_atomic(userringpage) should work. > > In short because there's no hardware involvement here, the established > mapping is just the pointer to the page, there is no need of setting > up any pagetables or to do any TLB flushes (except on 32bit archs if > the page is above the direct mapping but it never happens on 64bit > archs). Agree. The vmap is probably overkill if you only have a handfull of them kmap will be faster. Cheers, Jérôme