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Date:   Thu, 14 Apr 2022 09:55:36 +0300
From:   Alexey Dobriyan <adobriyan@gmail.com>
To:     Matthew Wilcox <willy@infradead.org>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        "Alex Xu (Hello71)" <alex_y_xu@yahoo.ca>,
        Daniel Colascione <dancol@google.com>,
        linux-api@vger.kernel.org, linux-fsdevel@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        Vlastimil Babka <vbabka@suse.cz>
Subject: Re: [PATCH] mm/smaps_rollup: return empty file for kthreads instead
 of ESRCH
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On Thu, Apr 14, 2022 at 04:23:13AM +0100, Matthew Wilcox wrote:
> On Wed, Apr 13, 2022 at 04:06:13PM -0700, Andrew Morton wrote:
> > On Wed, 13 Apr 2022 18:25:53 -0400 "Alex Xu (Hello71)" <alex_y_xu@yahoo.ca> wrote:
> > > > 258f669e7e88 was 4 years ago, so I guess a -stable backport isn't
> > > > really needed.
> > > 
> > > Current behavior (4.19+):
> [...]
> > > Pre-4.19 and post-patch behavior:
> > 
> > I don't think this will work very well.  smaps_rollup is the sort of
> > system tuning thing for which organizations will develop in-house
> > tooling which never get relesaed externally.
> > 
> > > 3. As mentioned previously, this was already the behavior between 4.14 
> > >    and 4.18 (inclusive).
> > > 
> > 
> > Yup.  Hm, tricky.  I'd prefer to leave it alone if possible.  How
> > serious a problem is this, really?  
> 
> I don't think "It's been like this for four years" is as solid an argument
> as you might like.  Certain distributions (of the coloured millinery
> variety, for example) haven't updated their kernel since then and so
> there may well be many organisations who have not been exposed to the
> current behaviour.  Even my employers distribution, while it offers a
> 5.4 based kernel, still has many customers who have not moved from the
> 4.14 kernel.  Inertia is a real thing, and restoring this older behaviour
> might well be an improvement.

Returning ESRCH is better so that programs don't waste time reading and
closing empty files and instantiating useless inodes.

Of course it is different if this patch was sent as response to a regression.