From mboxrd@z Thu Jan  1 00:00:00 1970
From: Matthew Wilcox <willy@infradead.org>
Subject: Re: [PATCH v18 31/32] mm: Add explicit page decrement in exception
 path for isolate_lru_pages
Date: Wed, 9 Sep 2020 21:15:30 +0100
Message-ID: <20200909201530.GM6583@casper.infradead.org>
References: <1598273705-69124-1-git-send-email-alex.shi@linux.alibaba.com>
 <1598273705-69124-32-git-send-email-alex.shi@linux.alibaba.com>
 <20200909010118.GB6583@casper.infradead.org>
 <CAKgT0UcjNx=00OgAQNWezc7UjLmF2NcDH0p7kzZ5D23PaFrFXA@mail.gmail.com>
 <alpine.LSU.2.11.2009091100280.9020@eggly.anvils>
Mime-Version: 1.0
Return-path: <linux-kernel-owner@vger.kernel.org>
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=Bw4rIJZLeK5WfU3TO9eMUF05YqQkLW/1cl+vzePNNf8=; b=iG6nBaZ2r0xjMc/5udrwMFgvVe
        zUDTE0FRIJKDA3f55Mj+WKsuMCkMrHGPVAgxs+CjEBF9K8DMTByr2eV/wyFcPvmVaNAeY9xTIqlOZ
        EH3nLSyN/oT93H3UyjXf1yValOLopcbpn3LFJsMi9LuSJgOzAT2F6nzyrkEgzOtPM85Kl/yajl+JY
        TL5tEsH6XBNI8KDUZBqbcFBznLuljUyY+vW9YlFRSz3iHlAQQ4a5F2JcCPtjiX1A+qizt0Ml+g35b
        CTKkt5fMZoPqXZNp7Sd7GqfcOLTiF/ol3aMbBxVvzoWXh/c3yqIjpa+1S7EGo6JxiWVpbEkf/03wb
        ciR5lr9g==;
Content-Disposition: inline
In-Reply-To: <alpine.LSU.2.11.2009091100280.9020@eggly.anvils>
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <cgroups.vger.kernel.org>
Content-Type: text/plain; charset="us-ascii"
Content-Transfer-Encoding: 7bit
To: Hugh Dickins <hughd@google.com>
Cc: Alexander Duyck <alexander.duyck@gmail.com>, Alex Shi <alex.shi@linux.alibaba.com>, Andrew Morton <akpm@linux-foundation.org>, Mel Gorman <mgorman@techsingularity.net>, Tejun Heo <tj@kernel.org>, Konstantin Khlebnikov <khlebnikov@yandex-team.ru>, Daniel Jordan <daniel.m.jordan@oracle.com>, Johannes Weiner <hannes@cmpxchg.org>, kbuild test robot <lkp@intel.com>, linux-mm <linux-mm@kvack.org>, LKML <linux-kernel@vger.kernel.org>, cgroups@vger.kernel.org, Shakeel Butt <shakeelb@google.com>, Joonsoo Kim <iamjoonsoo.kim@lge.com>, Wei Yang <richard.weiyang@gmail.com>, "Kirill A. Shutemov" <kirill@shutemov.name>, Rong Chen <rong.a.chen@intel.com>, Michal Hocko <mhocko@suse.com>, Vladimir Davydov <vdavydov.dev@gmail.com>, shy828301@gmail.com, Alexand

On Wed, Sep 09, 2020 at 11:24:14AM -0700, Hugh Dickins wrote:
> After overnight reflection, my own preference would be simply to
> drop this patch.  I think we are making altogether too much of a
> fuss here over what was simply correct as plain put_page()
> (and further from correct if we change it to leak the page in an
> unforeseen circumstance).
> 
> And if Alex's comment was not quite grammatically correct, never mind,
> it said as much as was worth saying.  I got more worried by his
> placement of the "busy:" label, but that does appear to work correctly.
> 
> There's probably a thousand places where put_page() is used, where
> it would be troublesome if it were the final put_page(): this one
> bothered you because you'd been looking at isolate_migratepages_block(),
> and its necessary avoidance of lru_lock recursion on put_page();
> but let's just just leave this put_page() as is.

My problem with put_page() is that it's no longer the simple
decrement-and-branch-to-slow-path-if-zero that it used to be.  It has the
awful devmap excrement in it so it really expands into a lot of code.
I really wish that "feature" could be backed out again.  It clearly
wasn't ready for merge.