linux-kernel - Re: [Patch v4] mm: thp: remove the defer list related code since this will not happen

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Date:   Sat, 18 Jan 2020 14:54:21 -0800
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Wei Yang <richardw.yang@...ux.intel.com>
Cc:     hannes@...xchg.org, mhocko@...nel.org, vdavydov.dev@...il.com,
        ktkhai@...tuozzo.com, kirill.shutemov@...ux.intel.com,
        yang.shi@...ux.alibaba.com, cgroups@...r.kernel.org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        alexander.duyck@...il.com, rientjes@...gle.com,
        stable@...r.kernel.org
Subject: Re: [Patch v4] mm: thp: remove the defer list related code since
 this will not happen

On Sat, 18 Jan 2020 07:38:36 +0800 Wei Yang <richardw.yang@...ux.intel.com> wrote:

> If compound is true, this means it is a PMD mapped THP. Which implies
> the page is not linked to any defer list. So the first code chunk will
> not be executed.
> 
> Also with this reason, it would not be proper to add this page to a
> defer list. So the second code chunk is not correct.
> 
> Based on this, we should remove the defer list related code.
> 
> Fixes: 87eaceb3faa5 ("mm: thp: make deferred split shrinker memcg aware")
> 
> Signed-off-by: Wei Yang <richardw.yang@...ux.intel.com>
> Suggested-by: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>
> Cc: <stable@...r.kernel.org>    [5.4+]

This patch is identical to "mm: thp: grab the lock before manipulating
defer list", which is rather confusing.  Please let people know when
this sort of thing is done.

The earlier changelog mentioned a possible race condition.  This
changelog does not.  In fact this changelog fails to provide any
description of any userspace-visible runtime effects of the bug. 
Please send along such a description for inclusion, as always.