Re: [PATCH] mm/pmem: Avoid inserting hugepage PTE entry with fsdax if hugepage support is disabled

[David Hildenbrand <david@redhat.com>]

On 05.02.21 03:39, Aneesh Kumar K.V wrote:
> Differentiate between hardware not supporting hugepages and user disabling THP
> via 'echo never > /sys/kernel/mm/transparent_hugepage/enabled'
> 
> For the devdax namespace, the kernel handles the above via the
> supported_alignment attribute and failing to initialize the namespace
> if the namespace align value is not supported on the platform.
> 
> For the fsdax namespace, the kernel will continue to initialize
> the namespace. This can result in the kernel creating a huge pte
> entry even though the hardware don't support the same.
> 
> We do want hugepage support with pmem even if the end-user disabled THP
> via sysfs file (/sys/kernel/mm/transparent_hugepage/enabled). Hence
> differentiate between hardware/firmware lacking support vs user-controlled
> disable of THP and prevent a huge fault if the hardware lacks hugepage
> support.
> 
> Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
> ---
>   include/linux/huge_mm.h | 15 +++++++++------
>   mm/huge_memory.c        |  6 +++++-
>   2 files changed, 14 insertions(+), 7 deletions(-)
> 
> diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
> index 6a19f35f836b..ba973efcd369 100644
> --- a/include/linux/huge_mm.h
> +++ b/include/linux/huge_mm.h
> @@ -78,6 +78,7 @@ static inline vm_fault_t vmf_insert_pfn_pud(struct vm_fault *vmf, pfn_t pfn,
>   }
>   
>   enum transparent_hugepage_flag {
> +	TRANSPARENT_HUGEPAGE_NEVER_DAX,
>   	TRANSPARENT_HUGEPAGE_FLAG,
>   	TRANSPARENT_HUGEPAGE_REQ_MADV_FLAG,
>   	TRANSPARENT_HUGEPAGE_DEFRAG_DIRECT_FLAG,
> @@ -123,6 +124,13 @@ extern unsigned long transparent_hugepage_flags;
>    */
>   static inline bool __transparent_hugepage_enabled(struct vm_area_struct *vma)
>   {
> +
> +	/*
> +	 * If the hardware/firmware marked hugepage support disabled.
> +	 */
> +	if (transparent_hugepage_flags & (1 << TRANSPARENT_HUGEPAGE_NEVER_DAX))
> +		return false;
> +
>   	if (vma->vm_flags & VM_NOHUGEPAGE)
>   		return false;
>   
> @@ -134,12 +142,7 @@ static inline bool __transparent_hugepage_enabled(struct vm_area_struct *vma)
>   
>   	if (transparent_hugepage_flags & (1 << TRANSPARENT_HUGEPAGE_FLAG))
>   		return true;
> -	/*
> -	 * For dax vmas, try to always use hugepage mappings. If the kernel does
> -	 * not support hugepages, fsdax mappings will fallback to PAGE_SIZE
> -	 * mappings, and device-dax namespaces, that try to guarantee a given
> -	 * mapping size, will fail to enable
> -	 */
> +
>   	if (vma_is_dax(vma))
>   		return true;
>   
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 9237976abe72..d698b7e27447 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -386,7 +386,11 @@ static int __init hugepage_init(void)
>   	struct kobject *hugepage_kobj;
>   
>   	if (!has_transparent_hugepage()) {
> -		transparent_hugepage_flags = 0;
> +		/*
> +		 * Hardware doesn't support hugepages, hence disable
> +		 * DAX PMD support.
> +		 */
> +		transparent_hugepage_flags = 1 << TRANSPARENT_HUGEPAGE_NEVER_DAX;
>   		return -EINVAL;
>   	}
>   
> 

Looks sane to me from my limited understanding of that code :)

-- 
Thanks,

David / dhildenb
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