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

[David Hildenbrand <david@redhat.com>]

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@redhat.com>
>> 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@redhat.com>
>> Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
>> Cc: Ryan Roberts <ryan.roberts@arm.com>
>> Cc: Christian Borntraeger <borntraeger@linux.ibm.com>
>> Cc: Janosch Frank <frankja@linux.ibm.com>
>> Cc: Claudio Imbrenda <imbrenda@linux.ibm.com>
>> Signed-off-by: David Hildenbrand <david@redhat.com>
>> ---
>>   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