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 13:36, Ryan Roberts wrote:
> On 11/10/2024 12:33, David Hildenbrand wrote:
>> 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? :)
> 
> Yeah fair enough, I was just thinking it makes the code simpler to keep all the
> checks in one place. But no strong opinion.
> 
> Either way:
> 
> Reviewed-by: Ryan Roberts <ryan.roberts@arm.com>

Thanks!

Also, I decided to not use "thp_vma_allowable_orders" because we are 
past the allocation phase (as indicated in the comment) and can really 
just change the way how we map the folio (PMD vs. PTE), not really 
*what* folio to use.

Ideally, in the future we have a different way of just saying "no PMD 
mappings please", decoupling the mapping from the allocation granularity.

-- 
Cheers,

David / dhildenb