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=-2.7 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,FREEMAIL_FORGED_FROMDOMAIN,FREEMAIL_FROM, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, URIBL_BLOCKED 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 2A5EBC433B4 for ; Fri, 9 Apr 2021 15:43:34 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 00A1861042 for ; Fri, 9 Apr 2021 15:43:33 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S233137AbhDIPnp (ORCPT ); Fri, 9 Apr 2021 11:43:45 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:44682 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229665AbhDIPno (ORCPT ); Fri, 9 Apr 2021 11:43:44 -0400 Received: from mail-ej1-x630.google.com (mail-ej1-x630.google.com [IPv6:2a00:1450:4864:20::630]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 93222C061760 for ; Fri, 9 Apr 2021 08:43:30 -0700 (PDT) Received: by mail-ej1-x630.google.com with SMTP id g17so6601139ejp.8 for ; Fri, 09 Apr 2021 08:43:30 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20161025; h=mime-version:references:in-reply-to:from:date:message-id:subject:to :cc; bh=eOQBkcBExWdOzHh9ROtfbTcad8EXL0NpCgTPDC8YVU0=; b=jHvR2b2XusmZqh290V+djJWa1Nxfumd3xyxS5ELiMJ/K0garq58cL8wPCxGzOxQc+1 f80QRSasdCvuaThHV6SywIF5ulzIzkl/PG83/Ev3BiezWm6AbIGBENwu3NKNJDAE55cb d+cl55dvnuuraoikehq4GnljwZ8YGdtAIJLOU451Nelo/mmp1xs4konHZZC8T9wybYly 8W05xf7jteaDKqyX7+Xt3Bc9Wo2gm24tDPc1ZZjLc1sXhBP56RPcufLHhMY8THHGXvGl YQYTkHBD0V2RGkYy28dzGrAu81XpAH7uAVE9GE++JahdYYj+0gD6jfehXgT4cn4KMp4x HWBQ== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:mime-version:references:in-reply-to:from:date :message-id:subject:to:cc; bh=eOQBkcBExWdOzHh9ROtfbTcad8EXL0NpCgTPDC8YVU0=; b=fBYQ52aL5P9gtd0uKzTesqrYrVsCDtwanW1FALDK++4ey8/qwE1dpzaPTtostfRBM2 qdjOmkrUWSDKOQRz49gnz7m7vPNdB45xW4omksD9PGbgfHT6rfvrrwc1bclGM44TcBKb BnffndLAJEgrX1iYK3ziJ4PUQEM5W2OOGOJccq/N1uxu7l+B3EyDC9yh1fV2D4y2LjQX JAwfaXF2KRf3NowbZFyP4Ml7QD88MKHv4T9rHazf+SR68HOGJyOISlITKJ6odKbrx455 j+nJ+bwo0Ji2B0T0DkjHlgc5Ad0QYTiMUBCuvh7CAr+8B9gUaSOamEnIXy9GP9FsDN9L edIQ== X-Gm-Message-State: AOAM5323qZefSDNhHGREZdGGFb32YA7bj59VdZXMoUF7FTD0Ry+YtBEi zYudFudTTYkPK8N4p9jU8ZL5O0a34EZtnZ3vheM= X-Google-Smtp-Source: ABdhPJxSRZFTfTZiEzUO2PcNytVm0YBqn2asyhjN/KVvOFgcYRdNlRK3rpo3nI4jFY4MyLWiNa/LP2FQSddharx7mAI= X-Received: by 2002:a17:906:4bce:: with SMTP id x14mr16335453ejv.383.1617983009301; Fri, 09 Apr 2021 08:43:29 -0700 (PDT) MIME-Version: 1.0 References: <20210401183216.443C4443@viggo.jf.intel.com> <20210401183223.80F1E291@viggo.jf.intel.com> In-Reply-To: From: Yang Shi Date: Fri, 9 Apr 2021 08:43:17 -0700 Message-ID: Subject: Re: [PATCH 04/10] mm/migrate: make migrate_pages() return nr_succeeded To: Oscar Salvador Cc: Dave Hansen , Linux MM , Linux Kernel Mailing List , Yang Shi , weixugc@google.com, Huang Ying , Dan Williams , David Hildenbrand Content-Type: text/plain; charset="UTF-8" Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Apr 8, 2021 at 10:06 PM Oscar Salvador wrote: > > On Thu, Apr 08, 2021 at 01:40:33PM -0700, Yang Shi wrote: > > Thanks a lot for the example code. You didn't miss anything. At first > > glance, I thought your suggestion seemed neater. Actually I > > misunderstood what Dave said about "That could really have caused some > > interesting problems." with multiple calls to migrate_pages(). I was > > thinking about: > > > > unsigned long foo() > > { > > unsigned long *ret_succeeded; > > > > migrate_pages(..., ret_succeeded); > > > > migrate_pages(..., ret_succeeded); > > > > return *ret_succeeded; > > } > > But that would not be a problem as well. I mean I am not sure what is > foo() supposed to do. > I assume is supposed to return the *total* number of pages that were > migrated? > > Then could do something like: > > unsigned long foo() > { > unsigned long ret_succeeded; > unsigned long total_succeeded = 0; > > migrate_pages(..., &ret_succeeded); > total_succeeded += ret_succeeded; > > migrate_pages(..., &ret_succeeded); > total_succeeded += ret_succeeded; > > return *total_succeeded; > } > > But AFAICS, you would have to do that with Wei Xu's version and with > mine, no difference there. It is because nr_succeeded is reset for each migrate_pages() call. You could do "*ret_succeeded += nr_succeeded" if we want an accumulated counter, then you don't have to add total_succeeded. And since nr_succeeded is reset for each migrate_pages() call, so both vm counter and trace point are happy. > > IIUC, Dave's concern was that nr_succeeded was only set to 0 at the beginning > of the function, and never reset back, which means, we would carry the > sum of previous nr_succeeded instead of the nr_succeeded in that round. > That would be misleading for e.g: reclaim in case we were to call > migrate_pages() several times, as instead of a delta value, nr_succeeded > would accumulate. I think the most straightforward concern is the vm counter and trace point in migrate_pages(), if migrate_pages() is called multiple times we may see messed up counters if nr_succeeded is not reset properly. Of course both your and Wei's suggestion solve this problem. But if we have usecase which returns nr_succeeded and call migrate_pages() multiple times, I think we do want to return accumulated value IMHO. > > But that won't happen neither with Wei Xu's version nor with mine. > > -- > Oscar Salvador > SUSE L3