Re: [PATCH v2] mm/hugetlb: Report -EHWPOISON not -EFAULT when FOLL_HWPOISON is specified

[Punit Agrawal <punit.agrawal@arm.com>]

James Morse <james.morse@arm.com> writes:

> KVM uses get_user_pages() to resolve its stage2 faults. KVM sets the
> FOLL_HWPOISON flag causing faultin_page() to return -EHWPOISON when it
> finds a VM_FAULT_HWPOISON. KVM handles these hwpoison pages as a special
> case.
>
> When huge pages are involved, this doesn't work so well. get_user_pages()
> calls follow_hugetlb_page(), which stops early if it receives
> VM_FAULT_HWPOISON from hugetlb_fault(), eventually returning -EFAULT to
> the caller. The step to map this to -EHWPOISON based on the FOLL_ flags
> is missing. The hwpoison special case is skipped, and -EFAULT is returned
> to user-space, causing Qemu or kvmtool to exit.
>
> Instead, move this VM_FAULT_ to errno mapping code into a header file
> and use it from faultin_page(), follow_hugetlb_page() and faultin_page().

You've got faultin_page() listed twice - I think you meant to add
fixup_user_fault().

With that addressed,

Acked-by: Punit Agrawal <punit.agrawal@arm.com>

>
> With this, KVM works as expected.
>
> CC: Punit Agrawal <punit.agrawal@arm.com>
> CC: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
> Signed-off-by: James Morse <james.morse@arm.com>
> ---
> Changes since v1:
>  * Fixed checkpatch warnings (oops, wrong file)
>  * Added vm_fault_to_errno() call to faultin_page() as Naoya Horiguchi
>    suggested.
>
> This isn't a problem for arm64 today as we haven't enabled MEMORY_FAILURE,
> but I can't see any reason this doesn't happen on x86 too, so I think this
> should be a fix. This doesn't apply earlier than stable's v4.11.1 due to
> all sorts of cleanup. My best offer is:
> Cc: <stable@vger.kernel.org> # 4.11.1
>
>  include/linux/mm.h | 11 +++++++++++
>  mm/gup.c           | 20 ++++++++------------
>  mm/hugetlb.c       |  5 +++++
>  3 files changed, 24 insertions(+), 12 deletions(-)
>
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 7cb17c6b97de..b892e95d4929 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -2327,6 +2327,17 @@ static inline struct page *follow_page(struct vm_area_struct *vma,
>  #define FOLL_REMOTE	0x2000	/* we are working on non-current tsk/mm */
>  #define FOLL_COW	0x4000	/* internal GUP flag */
>  
> +static inline int vm_fault_to_errno(int vm_fault, int foll_flags)
> +{
> +	if (vm_fault & VM_FAULT_OOM)
> +		return -ENOMEM;
> +	if (vm_fault & (VM_FAULT_HWPOISON | VM_FAULT_HWPOISON_LARGE))
> +		return (foll_flags & FOLL_HWPOISON) ? -EHWPOISON : -EFAULT;
> +	if (vm_fault & (VM_FAULT_SIGBUS | VM_FAULT_SIGSEGV))
> +		return -EFAULT;
> +	return 0;
> +}
> +
>  typedef int (*pte_fn_t)(pte_t *pte, pgtable_t token, unsigned long addr,
>  			void *data);
>  extern int apply_to_page_range(struct mm_struct *mm, unsigned long address,
> diff --git a/mm/gup.c b/mm/gup.c
> index d9e6fddcc51f..b3c7214d710d 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -407,12 +407,10 @@ static int faultin_page(struct task_struct *tsk, struct vm_area_struct *vma,
>  
>  	ret = handle_mm_fault(vma, address, fault_flags);
>  	if (ret & VM_FAULT_ERROR) {
> -		if (ret & VM_FAULT_OOM)
> -			return -ENOMEM;
> -		if (ret & (VM_FAULT_HWPOISON | VM_FAULT_HWPOISON_LARGE))
> -			return *flags & FOLL_HWPOISON ? -EHWPOISON : -EFAULT;
> -		if (ret & (VM_FAULT_SIGBUS | VM_FAULT_SIGSEGV))
> -			return -EFAULT;
> +		int err = vm_fault_to_errno(ret, *flags);
> +
> +		if (err)
> +			return err;
>  		BUG();
>  	}
>  
> @@ -723,12 +721,10 @@ int fixup_user_fault(struct task_struct *tsk, struct mm_struct *mm,
>  	ret = handle_mm_fault(vma, address, fault_flags);
>  	major |= ret & VM_FAULT_MAJOR;
>  	if (ret & VM_FAULT_ERROR) {
> -		if (ret & VM_FAULT_OOM)
> -			return -ENOMEM;
> -		if (ret & (VM_FAULT_HWPOISON | VM_FAULT_HWPOISON_LARGE))
> -			return -EHWPOISON;
> -		if (ret & (VM_FAULT_SIGBUS | VM_FAULT_SIGSEGV))
> -			return -EFAULT;
> +		int err = vm_fault_to_errno(ret, 0);
> +
> +		if (err)
> +			return err;
>  		BUG();
>  	}
>  
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index e5828875f7bb..3eedb187e549 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -4170,6 +4170,11 @@ long follow_hugetlb_page(struct mm_struct *mm, struct vm_area_struct *vma,
>  			}
>  			ret = hugetlb_fault(mm, vma, vaddr, fault_flags);
>  			if (ret & VM_FAULT_ERROR) {
> +				int err = vm_fault_to_errno(ret, flags);
> +
> +				if (err)
> +					return err;
> +
>  				remainder = 0;
>  				break;
>  			}

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