mm: don't install PMD mappings when THPs are disabled by the hw/process/vma

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

Posted by David Hildenbrand 1 year, 3 months ago

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")
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;
+
 	if (!thp_vma_suitable_order(vma, haddr, PMD_ORDER))
 		return ret;
 
-- 
2.46.1

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

Posted by Ryan Roberts 1 year, 3 months ago

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?

> 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()?

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

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

Posted by David Hildenbrand 1 year, 3 months ago

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

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

Posted by Ryan Roberts 1 year, 3 months ago

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>

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

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

Posted by David Hildenbrand 1 year, 3 months ago

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