Re: [PATCH v7 1/6] userfaultfd: add minor fault registration mode

[Mike Kravetz]

On 2/18/21 4:48 PM, Axel Rasmussen wrote:
<snip>
> @@ -401,8 +398,10 @@ vm_fault_t handle_userfault(struct vm_fault *vmf, unsigned long reason)
>  
>  	BUG_ON(ctx->mm != mm);
>  
> -	VM_BUG_ON(reason & ~(VM_UFFD_MISSING|VM_UFFD_WP));
> -	VM_BUG_ON(!(reason & VM_UFFD_MISSING) ^ !!(reason & VM_UFFD_WP));
> +	/* Any unrecognized flag is a bug. */
> +	VM_BUG_ON(reason & ~__VM_UFFD_FLAGS);
> +	/* 0 or > 1 flags set is a bug; we expect exactly 1. */
> +	VM_BUG_ON(!reason || !!(reason & (reason - 1)));

I may be confused, but that seems to be checking for a flag value of 1
as opposed to one flag being set?

>  
>  	if (ctx->features & UFFD_FEATURE_SIGBUS)
>  		goto out;
<snip>
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index 3bfba75f6cbd..0388107da4b1 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -4352,6 +4352,38 @@ static vm_fault_t hugetlb_no_page(struct mm_struct *mm,
>  				VM_FAULT_SET_HINDEX(hstate_index(h));
>  			goto backout_unlocked;
>  		}
> +
> +		/* Check for page in userfault range. */
> +		if (userfaultfd_minor(vma)) {
> +			u32 hash;
> +			struct vm_fault vmf = {
> +				.vma = vma,
> +				.address = haddr,
> +				.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
> +				 */
> +			};
> +
> +			unlock_page(page);
> +
> +			/*
> +			 * hugetlb_fault_mutex and i_mmap_rwsem must be dropped
> +			 * before handling userfault.  Reacquire after handling
> +			 * fault to make calling code simpler.
> +			 */
> +
> +			hash = hugetlb_fault_mutex_hash(mapping, idx);
> +			mutex_unlock(&hugetlb_fault_mutex_table[hash]);
> +			i_mmap_unlock_read(mapping);
> +			ret = handle_userfault(&vmf, VM_UFFD_MINOR);
> +			i_mmap_lock_read(mapping);
> +			mutex_lock(&hugetlb_fault_mutex_table[hash]);
> +			goto out;
> +		}
>  	}
>  
>  	/*
> 

I'm good with the hugetlb.c changes.  Since this in nearly identical to
the other handle_userfault() in this routine, it might be good to create
a common wrapper.  But, that is not required.
-- 
Mike Kravetz