Re: [PATCH 16/33] userfaultfd: hugetlbfs: add userfaultfd hugetlb hook

["Hillf Danton" <hillf.zj@alibaba-inc.com>]

> 
> From: Mike Kravetz <mike.kravetz@oracle.com>
> 
> When processing a hugetlb fault for no page present, check the vma to
> determine if faults are to be handled via userfaultfd.  If so, drop the
> hugetlb_fault_mutex and call handle_userfault().
> 
> Signed-off-by: Mike Kravetz <mike.kravetz@oracle.com>
> Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
> ---

Acked-by: Hillf Danton <hillf.zj@alibaba-inc.com> 

>  mm/hugetlb.c | 33 +++++++++++++++++++++++++++++++++
>  1 file changed, 33 insertions(+)
> 
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index baf7fd4..7247f8c 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -32,6 +32,7 @@
>  #include <linux/hugetlb.h>
>  #include <linux/hugetlb_cgroup.h>
>  #include <linux/node.h>
> +#include <linux/userfaultfd_k.h>
>  #include "internal.h"
> 
>  int hugepages_treat_as_movable;
> @@ -3589,6 +3590,38 @@ static int hugetlb_no_page(struct mm_struct *mm, struct vm_area_struct *vma,
>  		size = i_size_read(mapping->host) >> huge_page_shift(h);
>  		if (idx >= size)
>  			goto out;
> +
> +		/*
> +		 * Check for page in userfault range
> +		 */
> +		if (userfaultfd_missing(vma)) {
> +			u32 hash;
> +			struct fault_env fe = {
> +				.vma = vma,
> +				.address = address,
> +				.flags = flags,
> +				/*
> +				 * Hard to debug if it ends up being
> +				 * used by a callee that assumes
> +				 * something about the other
> +				 * uninitialized fields... same as in
> +				 * memory.c
> +				 */
> +			};
> +
> +			/*
> +			 * hugetlb_fault_mutex must be dropped before
> +			 * handling userfault.  Reacquire after handling
> +			 * fault to make calling code simpler.
> +			 */
> +			hash = hugetlb_fault_mutex_hash(h, mm, vma, mapping,
> +							idx, address);
> +			mutex_unlock(&hugetlb_fault_mutex_table[hash]);
> +			ret = handle_userfault(&fe, VM_UFFD_MISSING);
> +			mutex_lock(&hugetlb_fault_mutex_table[hash]);
> +			goto out;
> +		}
> +
>  		page = alloc_huge_page(vma, address, 0);
>  		if (IS_ERR(page)) {
>  			ret = PTR_ERR(page);
> 

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