Re: xfs: Xen/HPT related regression in v6.6

[Petr Vaněk]

On Mon, Nov 04, 2024 at 04:40:58PM +0100, David Hildenbrand wrote:
> On 04.11.24 15:11, Petr Vaněk wrote:
> > I would like to report a regression in XFS introduced in kerenel v6.6 in
> > commit 5d8edfb900d5 ("iomap: Copy larger chunks from userspace"). On a
> > system running under Xen, when a process creates a file on an XFS file
> > system and writes exactly 2MB or more in a single write syscall,
> > accessing memory through mmap on that file causes the process to hang,
> > while dmesg is flooded with page fault warnings:
> 
> [...]
> 
> > [   62.406493]  </TASK>
> > 
> > As shown in the log above, the issue persists in kernel 6.6.59. However,
> > it was recently resolved in commit 2b0f922323cc ("mm: don't install PMD
> > mappings when THPs are disabled by the hw/process/vma"). The fix was
> > backported to 6.11. Would it make sense to backport it to 6.6 as well?
> 
> I was speculating about this in the patch description:
> 
> "Is it also a problem when the HW disabled THP using 
> TRANSPARENT_HUGEPAGE_UNSUPPORTED?  At least on x86 this would be the 
> case without X86_FEATURE_PSE."
> 
> I assume we have a HW, where has_transparent_hugepage() == false, so 
> likely x86-64 without X86_FEATURE_PSE.
> 
> QEMU/KVM should be supporting X86_FEATURE_PSE, but maybe XEN does not 
> for its (PC?) guests? If I understood your setup correctly :)
> 
> At least years ago, this feature was not available in XEN PV guests [1].

Yes, as I understand it, the hugepages are not available in my Xen
guest.

> Note that I already sent a backport [2], I should probably ping at this 
> point.

Ah, I haven't noticed this one. It resolves the issue for me in 6.6.y.

> [1] https://lore.kernel.org/all/57188ED802000078000E431C@xxxxxxxxxxxxxxxxxxxxxxx/
> [2] https://lkml.kernel.org/r/20241022090952.4101444-1-david@xxxxxxxxxx

Thanks,
Petr