Re: [PATCH rfc -next 01/10] mm: add a generic VMA lock-based page fault handler

[Matthew Wilcox <willy@infradead.org>]

> +int try_vma_locked_page_fault(struct vm_locked_fault *vmlf, vm_fault_t *ret)
> +{
> +	struct vm_area_struct *vma;
> +	vm_fault_t fault;


On Thu, Jul 13, 2023 at 05:53:29PM +0800, Kefeng Wang wrote:
> +#define VM_LOCKED_FAULT_INIT(_name, _mm, _address, _fault_flags, _vm_flags, _regs, _fault_code) \
> +	_name.mm		= _mm;			\
> +	_name.address		= _address;		\
> +	_name.fault_flags	= _fault_flags;		\
> +	_name.vm_flags		= _vm_flags;		\
> +	_name.regs		= _regs;		\
> +	_name.fault_code	= _fault_code

More consolidated code is a good idea; no question.  But I don't think
this is the right way to do it.

> +int __weak arch_vma_check_access(struct vm_area_struct *vma,
> +				 struct vm_locked_fault *vmlf);

This should be:

#ifndef vma_check_access
bool vma_check_access(struct vm_area_struct *vma, )
{
	return (vma->vm_flags & vm_flags) == 0;
}
#endif

and then arches which want to do something different can just define
vma_check_access.

> +int try_vma_locked_page_fault(struct vm_locked_fault *vmlf, vm_fault_t *ret)
> +{
> +	struct vm_area_struct *vma;
> +	vm_fault_t fault;

Declaring the vmf in this function and then copying it back is just wrong.
We need to declare vm_fault_t earlier (in the arch fault handler) and
pass it in.  I don't think that creating struct vm_locked_fault is the
right idea either.

> +	if (!(vmlf->fault_flags & FAULT_FLAG_USER))
> +		return -EINVAL;
> +
> +	vma = lock_vma_under_rcu(vmlf->mm, vmlf->address);
> +	if (!vma)
> +		return -EINVAL;
> +
> +	if (arch_vma_check_access(vma, vmlf)) {
> +		vma_end_read(vma);
> +		return -EINVAL;
> +	}
> +
> +	fault = handle_mm_fault(vma, vmlf->address,
> +				vmlf->fault_flags | FAULT_FLAG_VMA_LOCK,
> +				vmlf->regs);
> +	*ret = fault;
> +
> +	if (!(fault & (VM_FAULT_RETRY | VM_FAULT_COMPLETED)))
> +		vma_end_read(vma);
> +
> +	if ((fault & VM_FAULT_RETRY))
> +		count_vm_vma_lock_event(VMA_LOCK_RETRY);
> +	else
> +		count_vm_vma_lock_event(VMA_LOCK_SUCCESS);
> +
> +	return 0;
> +}
> +
>  #endif /* CONFIG_PER_VMA_LOCK */
>  
>  #ifndef __PAGETABLE_P4D_FOLDED
> -- 
> 2.27.0
> 
>