Re: [PATCH v1 2/2] mm: don't install PMD mappings when THPs are disabled by the hw/process/vma

[David Hildenbrand]

On 11.10.24 13:29, Ryan Roberts wrote:
> On 11/10/2024 11:24, David Hildenbrand wrote:
> > We (or rather, readahead logic :) ) might be allocating a THP in the
> > pagecache and then try mapping it into a process that explicitly disabled
> > THP: we might end up installing PMD mappings.
> >
> > This is a problem for s390x KVM, which explicitly remaps all PMD-mapped
> > THPs to be PTE-mapped in s390_enable_sie()->thp_split_mm(), before
> > starting the VM.
> >
> > For example, starting a VM backed on a file system with large folios
> > supported makes the VM crash when the VM tries accessing such a mapping
> > using KVM.
> >
> > 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.
> >
> > In the future, we might be able to do better on s390x and only disallow
> > PMD mappings -- what s390x and likely TRANSPARENT_HUGEPAGE_UNSUPPORTED
> > really wants. For now, fix it by essentially performing the same check as
> > would be done in __thp_vma_allowable_orders() or in shmem code, where this
> > works as expected, and disallow PMD mappings, making us fallback to PTE
> > mappings.
> >
> > Reported-by: Leo Fu <bfu@xxxxxxxxxx>
> > Fixes: 793917d997df ("mm/readahead: Add large folio readahead")
>
> Will this patch be difficult to backport given it depends on the previous patch
> and that doesn't have a Fixes tag?

"difficult" -- not really. Andrew might want to tag patch #1  with 
"Fixes:" as well, but I can also send simple stable backports that avoid 
patch #1.

(Thinking again, I assume we want to Cc:stable)

> > Cc: Thomas Huth <thuth@xxxxxxxxxx>
> > Cc: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx>
> > Cc: Ryan Roberts <ryan.roberts@xxxxxxx>
> > Cc: Christian Borntraeger <borntraeger@xxxxxxxxxxxxx>
> > Cc: Janosch Frank <frankja@xxxxxxxxxxxxx>
> > Cc: Claudio Imbrenda <imbrenda@xxxxxxxxxxxxx>
> > Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
> > ---
> >   mm/memory.c | 9 +++++++++
> >   1 file changed, 9 insertions(+)
> >
> > diff --git a/mm/memory.c b/mm/memory.c
> > index 2366578015ad..a2e501489517 100644
> > --- a/mm/memory.c
> > +++ b/mm/memory.c
> > @@ -4925,6 +4925,15 @@ vm_fault_t do_set_pmd(struct vm_fault *vmf, struct page *page)
> >   	pmd_t entry;
> >   	vm_fault_t ret = VM_FAULT_FALLBACK;
> >   
> > +	/*
> > +	 * It is too late to allocate a small folio, we already have a large
> > +	 * folio in the pagecache: especially s390 KVM cannot tolerate any
> > +	 * PMD mappings, but PTE-mapped THP are fine. So let's simply refuse any
> > +	 * PMD mappings if THPs are disabled.
> > +	 */
> > +	if (thp_disabled_by_hw() || vma_thp_disabled(vma, vma->vm_flags))
> > +		return ret;
>
> Why not just call thp_vma_allowable_orders()?

Why call thp_vma_allowable_orders() that does a lot more work that 
doesn't really apply here? :)

I'd say, just like shmem, we handle this separately here.

--
Cheers,

David / dhildenb