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=-2.0 required=3.0 tests=DKIM_INVALID,DKIM_SIGNED, 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 8A8D0CA9EA0 for ; Fri, 25 Oct 2019 15:32:20 +0000 (UTC) Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by mail.kernel.org (Postfix) with ESMTP id 4B54521929 for ; Fri, 25 Oct 2019 15:32:19 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=fail reason="signature verification failed" (2048-bit key) header.d=infradead.org header.i=@infradead.org header.b="DI7Heg3R" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 4B54521929 Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=infradead.org Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=owner-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix) id 00CE86B0005; Fri, 25 Oct 2019 11:32:19 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id EFF656B0006; Fri, 25 Oct 2019 11:32:18 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id E15596B0007; Fri, 25 Oct 2019 11:32:18 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0138.hostedemail.com [216.40.44.138]) by kanga.kvack.org (Postfix) with ESMTP id BFCD36B0005 for ; Fri, 25 Oct 2019 11:32:18 -0400 (EDT) Received: from smtpin02.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay02.hostedemail.com (Postfix) with SMTP id 5FDD362E8 for ; Fri, 25 Oct 2019 15:32:18 +0000 (UTC) X-FDA: 76082698356.02.front87_4e4d18578ea5e X-HE-Tag: front87_4e4d18578ea5e X-Filterd-Recvd-Size: 5043 Received: from bombadil.infradead.org (bombadil.infradead.org [198.137.202.133]) by imf14.hostedemail.com (Postfix) with ESMTP for ; Fri, 25 Oct 2019 15:32:17 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=bombadil.20170209; h=In-Reply-To:Content-Type:MIME-Version :References:Message-ID:Subject:Cc:To:From:Date:Sender:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description:Resent-Date: Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID:List-Id: List-Help:List-Unsubscribe:List-Subscribe:List-Post:List-Owner:List-Archive; bh=+/gvmWPLED77Ax/dLgOjX8mNE1xoFxjC6a1H8EDfWLI=; b=DI7Heg3Rzw+oStT3b/lEoSSFU hZUtuX0mKVr3jNZkkAZE/ptp6HYkT5GkRd1yaOAJwpTmEVIr14js9eAdoPF90102RlY+y6GSupnKV FgUi5TK00zzM0kNJrIiNuswOq1UkVjLXMCTmvzqMkXutsJyta4VE+7reWjmy9nqXD70GrkYUfjIFG 67FXpRJD0V+qltjeOFImhWTnNBac2c+aaISiU0qB9/gatrM5xbSLVszuOGfXsc+5K5DEbqcj9HqWu +KSG29Sjk/lefD5Pd/SoyL5KBdx/d4zvpjMFdQzr4siCPMttMDq9mqXyJH4z5usGzfXihXZO+aEEG RcF61RLIQ==; Received: from willy by bombadil.infradead.org with local (Exim 4.92.3 #3 (Red Hat Linux)) id 1iO1ZN-0003Wd-Ux; Fri, 25 Oct 2019 15:32:05 +0000 Date: Fri, 25 Oct 2019 08:32:05 -0700 From: Matthew Wilcox To: Yang Shi Cc: hughd@google.com, aarcange@redhat.com, kirill.shutemov@linux.intel.com, gavin.dg@linux.alibaba.com, akpm@linux-foundation.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, stable@vger.kernel.org Subject: Re: [v4 PATCH] mm: thp: handle page cache THP correctly in PageTransCompoundMap Message-ID: <20191025153205.GQ2963@bombadil.infradead.org> References: <1571865575-42913-1-git-send-email-yang.shi@linux.alibaba.com> <20191024135547.GH2963@bombadil.infradead.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.12.1 (2019-06-15) 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 Thu, Oct 24, 2019 at 09:33:11AM -0700, Yang Shi wrote: > On 10/24/19 6:55 AM, Matthew Wilcox wrote: > > On Thu, Oct 24, 2019 at 05:19:35AM +0800, Yang Shi wrote: > > > We have usecase to use tmpfs as QEMU memory backend and we would like to > > > take the advantage of THP as well. But, our test shows the EPT is not > > > PMD mapped even though the underlying THP are PMD mapped on host. > > > The number showed by /sys/kernel/debug/kvm/largepage is much less than > > > the number of PMD mapped shmem pages as the below: > > > > > > 7f2778200000-7f2878200000 rw-s 00000000 00:14 262232 /dev/shm/qemu_back_mem.mem.Hz2hSf (deleted) > > > Size: 4194304 kB > > > [snip] > > > AnonHugePages: 0 kB > > > ShmemPmdMapped: 579584 kB > > > [snip] > > > Locked: 0 kB > > > > > > cat /sys/kernel/debug/kvm/largepages > > > 12 > > > > > > And some benchmarks do worse than with anonymous THPs. > > > > > > By digging into the code we figured out that commit 127393fbe597 ("mm: > > > thp: kvm: fix memory corruption in KVM with THP enabled") checks if > > > there is a single PTE mapping on the page for anonymous THP when > > > setting up EPT map. But, the _mapcount < 0 check doesn't fit to page > > > cache THP since every subpage of page cache THP would get _mapcount > > > inc'ed once it is PMD mapped, so PageTransCompoundMap() always returns > > > false for page cache THP. This would prevent KVM from setting up PMD > > > mapped EPT entry. > > > > > > So we need handle page cache THP correctly. However, when page cache > > > THP's PMD gets split, kernel just remove the map instead of setting up > > > PTE map like what anonymous THP does. Before KVM calls get_user_pages() > > > the subpages may get PTE mapped even though it is still a THP since the > > > page cache THP may be mapped by other processes at the mean time. > > > > > > Checking its _mapcount and whether the THP has PTE mapped or not. > > > Although this may report some false negative cases (PTE mapped by other > > > processes), it looks not trivial to make this accurate. > > I don't understand why you care how it's mapped into userspace. If there > > is a PMD-sized page in the page cache, then you can use a PMD mapping > > in the EPT tables to map it. Why would another process having a PTE > > mapping on the page cause you to not use a PMD mapping? > > We don't care if the THP is PTE mapped by other process, but either > PageDoubleMap flag or _mapcount/compound_mapcount can't tell us if the PTE > map comes from the current process or other process unless gup could return > pmd's status. But why do you care if the THP is PTE mapped by _this_ process? This process has a reference to the page; the page is PMD sized and PMD aligned, so you can use a PMD mapping in the guest, regardless of how it's mapped by userspace. Maybe this process doesn't even have the page mapped at all?