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

[Barry K. Nathan]

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@xxxxxxxxx>