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

[David Hildenbrand <david@redhat.com>]

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].


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

[1] 
https://lore.kernel.org/all/57188ED802000078000E431C@prv-mh.provo.novell.com/
[2] https://lkml.kernel.org/r/20241022090952.4101444-1-david@redhat.com

> 
> I encountered this issue while updating a Gentoo VM with an XFS
> filesystem running under Xen. During a final stage of glibc update,
> files were copied to the live system, but when locale-gen started, it
> hung. I couldn't open a new shell, as it attempted to mmap an
> LC_COLLATE-related file, resulting in the same page faults as reported
> above.

Yes, looks like something is not happy about the PMD mapping that we 
installed.

-- 
Cheers,

David / dhildenb