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=-8.5 required=3.0 tests=BAYES_00,DKIM_INVALID, DKIM_SIGNED,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, MENTIONS_GIT_HOSTING,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED 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 CC9FDC43461 for ; Wed, 9 Sep 2020 17:04:19 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 61B4121919 for ; Wed, 9 Sep 2020 17:04:19 +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="PTtQzQuk" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1730673AbgIIRER (ORCPT ); Wed, 9 Sep 2020 13:04:17 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:33964 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1730551AbgIIREK (ORCPT ); Wed, 9 Sep 2020 13:04:10 -0400 Received: from casper.infradead.org (casper.infradead.org [IPv6:2001:8b0:10b:1236::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 13401C061573; Wed, 9 Sep 2020 10:04:10 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=casper.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; bh=LfHxT4ZNG1X4ZvN2dtQwo2pwpei3uAvCD6OoC4DuCCA=; b=PTtQzQukf2s/Vju2X06eMGOIWP yiKhMDLhWzkhhzXfGg5yOClIpORVoMG0/fj6/fpwo52WI1xZPN5qKbPPCsMiNn0HPycb4loydQe7S I6WrDdx271lhA3YooKU3xJDSgsMCZyHvpo86ehdFhVeKuyESTJNrwTEiRPQOwJfafit1oD5PsZCgx MFxdeLOJPKdwBNxs+qH9oG9W3ybCDan9eMHA3eGFa0liIqhXormAofOtqmu5+vGxHN3/rPBfYcXRs ys5s3+t08k47reP+/evgeFtoXj55NgSu/kMDsPjPinUWRrwZmVzZwdHRTCZ2HLoSovpn8vQAS2/pI +qSPo5CQ==; Received: from willy by casper.infradead.org with local (Exim 4.92.3 #3 (Red Hat Linux)) id 1kG3Vq-0007vP-FL; Wed, 09 Sep 2020 17:04:03 +0000 Date: Wed, 9 Sep 2020 18:04:02 +0100 From: Matthew Wilcox To: Christoph Hellwig Cc: Ming Mao , linux-kernel@vger.kernel.org, kvm@vger.kernel.org, linux-mm@kvack.org, alex.williamson@redhat.com, akpm@linux-foundation.org, cohuck@redhat.com, jianjay.zhou@huawei.com, weidong.huang@huawei.com, peterx@redhat.com, aarcange@redhat.com, wangyunjian@huawei.com, jhubbard@nvidia.com Subject: Re: [PATCH V4 1/2] vfio dma_map/unmap: optimized for hugetlbfs pages Message-ID: <20200909170402.GJ6583@casper.infradead.org> References: <20200908133204.1338-1-maoming.maoming@huawei.com> <20200908133204.1338-2-maoming.maoming@huawei.com> <20200909080114.GA8321@infradead.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200909080114.GA8321@infradead.org> Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Sep 09, 2020 at 09:01:14AM +0100, Christoph Hellwig wrote: > (1) a FOLL_HUGEPAGE flag for the pin_user_pages API family that returns > a single struct page for any kind of huge page, which would also > benefit all kinds of other users rather than adding these kinds of > hacks to vfio. > (2) add a bvec version of the API that returns a variable size > "extent" > > I had started on (2) a while ago, and here is branch with my code (which > is broken and fails test, but might be a start): > > http://git.infradead.org/users/hch/misc.git/shortlog/refs/heads/gup-bvec > > But for now I wonder if (1) is the better start, which could still be > reused to (2) later. (1) doesn't work. Suppose you have a 16kB MAP_PRIVATE of a file which happens to have a THP in the page cache. When you write a byte, that base page gets copied and your bvec for that 16kB needs to look something like: { page A, offset 0, length 4096 }, { page B, offset 0, length 4096 }, { page A, offset 8192, length 8192 }, You can come up with other scenarios, like partial mappings of THPs with an adjacent VMA of a different THP, but I think we just need to make (2) work.