linux-kernel - Re: [PATCH] mm/smaps_rollup: return empty file for kthreads instead of ESRCH

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Date:   Thu, 14 Apr 2022 09:38:14 +0200
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Alexey Dobriyan <adobriyan@...il.com>,
        Matthew Wilcox <willy@...radead.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        "Alex Xu (Hello71)" <alex_y_xu@...oo.ca>,
        Daniel Colascione <dancol@...gle.com>,
        linux-api@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH] mm/smaps_rollup: return empty file for kthreads instead
 of ESRCH

On 4/14/22 08:55, Alexey Dobriyan wrote:
> 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@...oo.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.

Hm, unfortunately I don't remember why I put return -ESRCH for this case in
addition to get_proc_task() failing. I doubt it was a conscious decision to
treat kthreads differently - I think I would have preferred consistency with
maps/smaps.

Can the awk use case be fixed with some flag to make it ignore the errors?

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