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

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

On Thu, Jun 26, 2025 at 01:59:32AM -0700, Barry K. Nathan wrote:
> 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.

Thank you!  I'll go queue that up now and push out a new -rc release

greg k-h