linux-kernel - Re: [PATCH v2] mm/page_alloc: fix memalloc_nocma

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Message-Id: <20200723201404.f76ccd792c9e74f3ae8ec9f5@linux-foundation.org>
Date:   Thu, 23 Jul 2020 20:14:04 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Joonsoo Kim <js1304@...il.com>
Cc:     Linux Memory Management List <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>, kernel-team@....com,
        Vlastimil Babka <vbabka@...e.cz>,
        Christoph Hellwig <hch@...radead.org>,
        Roman Gushchin <guro@...com>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
        Michal Hocko <mhocko@...e.com>,
        "Aneesh Kumar K . V" <aneesh.kumar@...ux.ibm.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>, stable@...r.kernel.org
Subject: Re: [PATCH v2] mm/page_alloc: fix memalloc_nocma_{save/restore}
 APIs

On Fri, 24 Jul 2020 12:04:02 +0900 Joonsoo Kim <js1304@...il.com> wrote:

> 2020년 7월 24일 (금) 오전 11:36, Andrew Morton <akpm@...ux-foundation.org>님이 작성:
> >
> > On Fri, 24 Jul 2020 11:23:52 +0900 Joonsoo Kim <js1304@...il.com> wrote:
> >
> > > > > Second, clearing __GFP_MOVABLE in current_gfp_context() has a side effect
> > > > > to exclude the memory on the ZONE_MOVABLE for allocation target.
> > > >
> > > > More whoops.
> > > >
> > > > Could we please have a description of the end-user-visible effects of
> > > > this change?  Very much needed when proposing a -stable backport, I think.
> > >
> > > In fact, there is no noticeable end-user-visible effect since the fallback would
> > > cover the problematic case. It's mentioned in the commit description. Perhap,
> > > performance would be improved due to reduced retry and more available memory
> > > (we can use ZONE_MOVABLE with this patch) but it would be neglectable.
> > >
> > > > d7fefcc8de9147c is over a year old.  Why did we only just discover
> > > > this?  This makes one wonder how serious those end-user-visible effects
> > > > are?
> > >
> > > As mentioned above, there is no visible problem to the end-user.
> >
> > OK, thanks.  In that case, I don't believe that a stable backport is
> > appropriate?
> >
> > (Documentation/process/stable-kernel-rules.rst)
> 
> Thanks for the pointer!
> 
> Hmm... I'm not sure the correct way to handle this patch. I thought that
> memalloc_nocma_{save,restore} is an API that is callable from the module.
> If it is true, it's better to regard this patch as the stable candidate since
> out-of-tree modules could use it without the fallback and it would cause
> a problem. But, yes, there is no visible problem to the end-user, at least,
> within the mainline so it is possibly not a stable candidate.
> 
> Please share your opinion about this situation.

We tend not to care much about out-of-tree modules.  I don't think a
theoretical concern for out-of-tree code justifies risking the
stability of -stable kernels.