linux-kernel - Re: [PATCH v18 31/32] mm: Add explicit page decrement in exception path for isolate_lru

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Date:   Wed, 9 Sep 2020 14:05:20 -0700 (PDT)
From:   Hugh Dickins <hughd@...gle.com>
To:     Matthew Wilcox <willy@...radead.org>
cc:     Hugh Dickins <hughd@...gle.com>,
        Alexander Duyck <alexander.duyck@...il.com>,
        Alex Shi <alex.shi@...ux.alibaba.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Tejun Heo <tj@...nel.org>,
        Konstantin Khlebnikov <khlebnikov@...dex-team.ru>,
        Daniel Jordan <daniel.m.jordan@...cle.com>,
        Johannes Weiner <hannes@...xchg.org>,
        kbuild test robot <lkp@...el.com>,
        linux-mm <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>, cgroups@...r.kernel.org,
        Shakeel Butt <shakeelb@...gle.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Wei Yang <richard.weiyang@...il.com>,
        "Kirill A. Shutemov" <kirill@...temov.name>,
        Rong Chen <rong.a.chen@...el.com>,
        Michal Hocko <mhocko@...e.com>,
        Vladimir Davydov <vdavydov.dev@...il.com>, shy828301@...il.com,
        Alexander Duyck <alexander.h.duyck@...ux.intel.com>
Subject: Re: [PATCH v18 31/32] mm: Add explicit page decrement in exception
 path for isolate_lru_pages

On Wed, 9 Sep 2020, Matthew Wilcox wrote:
> 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.

And I suppose I should thank you for opening my eyes to that.
I knew there was "dev" stuff inside __put_page(), but didn't
realize that the inline put_page() has now been defiled.
Yes, I agree, that is horrid and begs to be undone.

But this is not the mail thread for discussing that, and we should
not use strange alternatives to put_page(), here or elsewhere,
just to avoid that (surely? hopefully?) temporary excrescence.

Hugh