linux-kernel - Re: [PATCH 6.15 000/588] 6.15.4-rc2 review

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Message-ID: <6d3b0f28-98cb-46dd-b971-8a11e3b69d68@pobox.com>
Date: Wed, 25 Jun 2025 05:15:21 -0700
From: "Barry K. Nathan" <barryn@...ox.com>
To: Holger Hoffstätte <holger@...lied-asynchrony.com>,
 Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc: stable@...r.kernel.org, patches@...ts.linux.dev,
 linux-kernel@...r.kernel.org, torvalds@...ux-foundation.org,
 akpm@...ux-foundation.org, linux@...ck-us.net, shuah@...nel.org,
 patches@...nelci.org, lkft-triage@...ts.linaro.org, pavel@...x.de,
 jonathanh@...dia.com, f.fainelli@...il.com, sudipm.mukherjee@...il.com,
 srw@...dewatkins.net, rwarsow@....de, conor@...nel.org,
 hargar@...rosoft.com, broonie@...nel.org,
 Christian Brauner <brauner@...nel.org>
Subject: Re: [PATCH 6.15 000/588] 6.15.4-rc2 review

On 6/25/25 02:08, Holger Hoffstätte wrote:
> On 2025-06-25 10:25, Greg Kroah-Hartman wrote:
>> On Wed, Jun 25, 2025 at 10:00:56AM +0200, Holger Hoffstätte wrote:
>>> (cc: Christian Brauner>
>>>
>>> Since 6.15.4-rc1 I noticed that some KDE apps (kded6, kate (the text 
>>> editor))
>>> started going into a tailspin with 100% per-process CPU.
>>>
>>> The symptom is 100% reproducible: open a new file with kate, save 
>>> empty file,
>>> make changes, save, watch CPU go 100%. perf top shows copy_to_user 
>>> running wild.
>>>
>>> First I tried to reproduce on 6.15.3 - no problem, everything works 
>>> fine.
>>>
>>> After checking the list of patches for 6.15.4 I reverted the 
>>> anon_inode series
>>> (all 3 for the first attempt) and the problem is gone.
>>>
>>> Will try to reduce further & can gladly try additional fixes, but for 
>>> now
>>> I'd say these patches are not yet suitable for stable.
>>
>> Does this same issue also happen for you on 6.16-rc3?
> 
> Curiously it does *not* happen on 6.16-rc3, so that's good.
> I edited/saved several files and everything works as it should.
> 
> In 6.15.4-rc the problem occurs (as suspected) with:
> anon_inode-use-a-proper-mode-internally.patch aka cfd86ef7e8e7 upstream.
> 
> thanks
> Holger

For what it's worth, I can confirm this reproduces easily and 
consistently on Debian trixie's KDE (6.3.5), with either Wayland or X11. 
It reproduces with kernel 6.15.4-rc2, and with 
6.15.3+anon_inode-use-a-proper-mode-internally.patch, but not with 
vanilla 6.15.3 or with 6.16-rc3.

By the way, my test VM has both GNOME and KDE installed. If I boot one 
of the affected kernels and log into a GNOME session, I don't get any 
GNOME processes chewing up CPU the way that some of the KDE processes 
do. However, if I then start kate within the GNOME session and follow 
the steps to reproduce (create a new file, save it immediately, type a 
few characters, save again), kate still starts using 100% CPU.
-- 
-Barry K. Nathan  <barryn@...ox.com>