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

[Ryan Roberts]

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?

> 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()?

> +
>  	if (!thp_vma_suitable_order(vma, haddr, PMD_ORDER))
>  		return ret;
>