Re: [PATCH] sparc64: fix hugetlb for sun4u

[John Paul Adrian Glaubitz]

Hi David,

thanks a lot for your input!

On Mon, 2025-08-11 at 10:20 +0200, David Hildenbrand wrote:
> On 09.08.25 08:23, John Paul Adrian Glaubitz wrote:
> > Hi Anthony,
> > 
> > On Sat, 2025-08-09 at 00:37 +0200, John Paul Adrian Glaubitz wrote:
> > > > Maybe try enabling CONFIG_DEBUG_VM_IRQSOFF, CONFIG_DEBUG_VM, and perhaps
> > > > CONFIG_DEBUG_VM_PGFLAGS. That might help detect a problem closer to the
> > > > source. You can also try adding transparent_hugepage=never to the kernel
> > > > boot line to see if compiling in THP support but not using it is okay.
> > > 
> > > OK, I will try that. But not today anymore. It's half past midnight now here in Germany
> > > and I was debugging this issue almost all day long. I'm glad to have finally been able
> > > to track this down to THP support being enabled.
> > > 
> > > Maybe you can try whether you can reproduce this in QEMU as well.
> > 
> > OK, first data point: Setting CONFIG_TRANSPARENT_HUGEPAGE_NEVER=y causes the backtrace during
> > boot to disappear with CONFIG_TRANSPARENT_HUGEPAGE=y. However, it still disappears later when
> > running "apt update && apt -y upgrade" again:
> 
> Just to give some context: (m)THPs in file systems will get used 
> independently of CONFIG_TRANSPARENT_HUGEPAGE_NEVER=y.
> 
> So CONFIG_TRANSPARENT_HUGEPAGE_NEVER=y primarily only controls usage of 
> (m)THPs for anonymous memory, but not in the pagecache.

Thanks, this actually explains why CONFIG_TRANSPARENT_HUGEPAGE_NEVER=y doesn't fully mitigate
the problem.

> > [  170.472743] kernel BUG at fs/ext4/inode.c:1174!
> > [  170.532313]               \|/ ____ \|/
> >                               "@'/ .. \`@"
> >                               /_| \__/ |_\
> >                                  \__U_/
> 
> Is this the
> 
> BUG_ON(to > folio_size(folio));

Yes, I just looked it up in my current local kernel tree and that's indeed the line.

Adrian

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