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 Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by smtp.lore.kernel.org (Postfix) with ESMTP id 9D640ECAAD1 for ; Wed, 31 Aug 2022 21:42:24 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id D3B896B0072; Wed, 31 Aug 2022 17:42:23 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id CEBC56B0073; Wed, 31 Aug 2022 17:42:23 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id BB2848D0001; Wed, 31 Aug 2022 17:42:23 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from relay.hostedemail.com (smtprelay0015.hostedemail.com [216.40.44.15]) by kanga.kvack.org (Postfix) with ESMTP id AAC006B0072 for ; Wed, 31 Aug 2022 17:42:23 -0400 (EDT) Received: from smtpin18.hostedemail.com (a10.router.float.18 [10.200.18.1]) by unirelay08.hostedemail.com (Postfix) with ESMTP id 6F6651414BE for ; Wed, 31 Aug 2022 21:42:23 +0000 (UTC) X-FDA: 79861211766.18.8D8C1C6 Received: from mail-pj1-f46.google.com (mail-pj1-f46.google.com [209.85.216.46]) by imf13.hostedemail.com (Postfix) with ESMTP id 1192E20012 for ; Wed, 31 Aug 2022 21:42:22 +0000 (UTC) Received: by mail-pj1-f46.google.com with SMTP id mj6so10305864pjb.1 for ; Wed, 31 Aug 2022 14:42:22 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20210112; h=cc:to:subject:message-id:date:from:in-reply-to:references :mime-version:from:to:cc:subject:date; bh=NjRf8u0mv8cKmfOvGyjwyOrZtUjHTYSkAjDvlc9u+is=; b=YX4XnAZSJXkF5SYuEfT3jHkdzOmE23LlWh6dZ/ZRleW7y77ngN7nIKtUC9g1YoiDLV hZ6Nufm3YYWjgCe/3astrxymHmJHucLvSBliARFA+a+lrwwWtVrV61Rn435PtCJOt182 TyeSB9ND6qkGhboFhboiRuHNHPRfZUHSz2s5y3CrUqNy6/Inll2JYbiNXWbp/Z9wQ8ln BJYXkDdCHzc7rxlp9WJBDBeNep0UM2fR80L4TYcgUR1vPAmRYD14LjID+yq3mEc84Isu jiFoXDn1sCk48Uzh/yYYmG6zkmHByAqB1xTvnhnvxVEq2TXG2zS6Vda5nKWr0HQvjWC9 9H5w== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=cc:to:subject:message-id:date:from:in-reply-to:references :mime-version:x-gm-message-state:from:to:cc:subject:date; bh=NjRf8u0mv8cKmfOvGyjwyOrZtUjHTYSkAjDvlc9u+is=; b=sG2N/B/2hTifsV7K7T1VRQOBAlRhcKi1KprQPltWF78Qf+fE682AGs2vX/cqmRr175 wLyXLRrVwBT2kBN7MZbWhaQlOlnfpWki9MUs15XQK95k6+5Eqlh1DYv4c4dzYWV+3y2M yWyHAhFZMTgn8a/NLsdocdNR87k38SVPR3u7W7oDlQil3pK/JkNqz2835ooksO7pr1Bj EdRVft78bfouxZggXJahogjvETpPm1vQvTZzXnalVbJKvvFUWcuIetu4Zum2A7FDDlbW V6VMFWJPLQ76vYpObQNmJ6VrWH99N2ySl7NVqR1IhGeOlPiTsrBZ+8/E5h88S0B6IemI qS+g== X-Gm-Message-State: ACgBeo0FvwsjQyQ+wjZkYWO0OwXXzQzQd5b4Wxo8oTrKI7zKkoM61P4W WTHKhCBx2gFOlfxyDeszs1BuNqxvxshcshqMaSQ= X-Google-Smtp-Source: AA6agR5NjvFV+cbhRtZDeg48AkDYYDIq/0bwkPcwCkqMtNBBOCGP27YnSD+rCJ6IvL/JOkr2wrckQgFsFdtUTYhTlL4= X-Received: by 2002:a17:903:32d0:b0:174:ce51:223f with SMTP id i16-20020a17090332d000b00174ce51223fmr16296481plr.117.1661982142031; Wed, 31 Aug 2022 14:42:22 -0700 (PDT) MIME-Version: 1.0 References: <20220831083024.37138-1-david@redhat.com> <4845ae71-b7dd-1707-ebc3-2eb3521e7fa0@redhat.com> <94c3217d-df73-2b6b-21f0-95baf117c584@redhat.com> In-Reply-To: From: Yang Shi Date: Wed, 31 Aug 2022 14:42:09 -0700 Message-ID: Subject: Re: [PATCH v1] mm/ksm: update stale comment in write_protect_page() To: John Hubbard Cc: David Hildenbrand , linux-kernel@vger.kernel.org, linux-mm@kvack.org, Andrew Morton , Jason Gunthorpe , Andrea Arcangeli , Hugh Dickins , Peter Xu , "Kirill A. Shutemov" Content-Type: text/plain; charset="UTF-8" ARC-Authentication-Results: i=1; imf13.hostedemail.com; dkim=pass header.d=gmail.com header.s=20210112 header.b=YX4XnAZS; spf=pass (imf13.hostedemail.com: domain of shy828301@gmail.com designates 209.85.216.46 as permitted sender) smtp.mailfrom=shy828301@gmail.com; dmarc=pass (policy=none) header.from=gmail.com ARC-Seal: i=1; s=arc-20220608; d=hostedemail.com; t=1661982143; a=rsa-sha256; cv=none; b=wsGPfaSMJgqywtDSMmMiwDZppx3P1xBDhbDvyt3HxgFlTJe61iu17t0smUc8eoP4Lw4LTz iVk611Ld2GxoRXyE2sy1gX4RDNndoY0teJ3uKszBuLzvHRCatTH+CK5hlL/oEC6Ik6evCE 7hjc0+tEFtHsTAa4ocCEpPSCL9Om9Vw= ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1661982143; h=from:from:sender:reply-to:subject:subject:date:date: message-id:message-id:to:to:cc:cc:mime-version:mime-version: content-type:content-type:content-transfer-encoding: in-reply-to:in-reply-to:references:references:dkim-signature; bh=NjRf8u0mv8cKmfOvGyjwyOrZtUjHTYSkAjDvlc9u+is=; b=4QZlzzl/knmxx5gO+hYV6YtxVZaXn7NwhEom/AgUO+G3W1XcJ9XkijKDSR05hGf3zsWSHY EJCfqU/JRFUiM9EKFHOs51//3llferC/0a4zorH56SQg+I/CzspMuVAsOh6pejG20r5Qcf 4qsMBy2IuRdR6FQythJFg+eOa55gaBI= X-Rspamd-Server: rspam02 X-Rspamd-Queue-Id: 1192E20012 X-Rspam-User: Authentication-Results: imf13.hostedemail.com; dkim=pass header.d=gmail.com header.s=20210112 header.b=YX4XnAZS; spf=pass (imf13.hostedemail.com: domain of shy828301@gmail.com designates 209.85.216.46 as permitted sender) smtp.mailfrom=shy828301@gmail.com; dmarc=pass (policy=none) header.from=gmail.com X-Stat-Signature: p8nz6t77auqnj8k3g55wqahd8by9z4bp X-HE-Tag: 1661982142-271117 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 Wed, Aug 31, 2022 at 1:59 PM John Hubbard wrote: > > On 8/31/22 12:43, Yang Shi wrote: > >>> It looks powerpc does issue IPI for pmd flush. But arm64 doesn't IIRC. > >>> > >>> So maybe we should implement pmdp_collapse_flush() for those arches to > >>> issue IPI. > >> > >> ... or find another way to detect and handle this in GUP-fast? > >> > >> Not sure if, for handling PMDs, it could be sufficient to propagate the > >> pmdp pointer + value and double check that the values didn't change. > > > > Should work too, right before pinning the page. > > > > pmdp_collapse_flush() is actually just called by khugepaged, so arch > > specific implementation should not be a problem and we avoid making > > gup fast more complicated. > > > > And just to pile on, about that gup fast complexity: depending upon IPIs > added a lot of complexity, not just because of the IPI dependency, but > more importantly because only some arches even *have* IPIs. So an > entirely different set of reasoning has to be used *in addition* to > working through the IPI story. And sure enough, we can see the fallout: > you are uncovering lots of half-correct comments in that area. > > So getting rid of the dependency on IPIs in gup fast would go a long way > to simplifying it, and maybe even improving overall CPU load (insert > some hand-wavy notes here about IPIs being worse than things like RCU). > > But the real win is in the complexity reduction in gup fast. Thanks, John. Yeah, I still had some wrong impressions about how to serialize against fast GUP. If you guys thought fixing the problem in gup code is the preferred way, I won't insist on arch-specific pmdp_collapse_flush(). > > > thanks, > > -- > John Hubbard > NVIDIA