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

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Message-ID: <7069cc77-a224-4753-8088-0302fc444710@pobox.com>
Date: Thu, 26 Jun 2025 01:59:32 -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 05:15, Barry K. Nathan wrote:
> 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.

After some testing and bisecting, I found that "anon_inode: use a proper 
mode internally" needs to be followed up with "fs: add S_ANON_INODE" 
(upstream commit 19bbfe7b5fcc) in order to avoid this regression.
-- 
-Barry K. Nathan  <barryn@...ox.com>