Re: [PATCH rfc v2 01/10] mm: add a generic VMA lock-based page fault handler

[Kefeng Wang]

On 2023/8/24 15:12, Alexander Gordeev wrote:
> On Mon, Aug 21, 2023 at 08:30:47PM +0800, Kefeng Wang wrote:
>
> Hi Kefeng,
>
> > The ARCH_SUPPORTS_PER_VMA_LOCK are enabled by more and more architectures,
> > eg, x86, arm64, powerpc and s390, and riscv, those implementation are very
> > similar which results in some duplicated codes, let's add a generic VMA
> > lock-based page fault handler try_to_vma_locked_page_fault() to eliminate
> > them, and which also make us easy to support this on new architectures.
> >
> > Since different architectures use different way to check vma whether is
> > accessable or not, the struct pt_regs, page fault error code and vma flags
> > are added into struct vm_fault, then, the architecture's page fault code
> > could re-use struct vm_fault to record and check vma accessable by each
> > own implementation.
> >
> > Signed-off-by: Kefeng Wang <wangkefeng.wang@xxxxxxxxxx>
> > ---
...
> > +
> > +vm_fault_t try_vma_locked_page_fault(struct vm_fault *vmf)
> > +{
> > +	vm_fault_t fault = VM_FAULT_NONE;
> > +	struct vm_area_struct *vma;
> > +
> > +	if (!(vmf->flags & FAULT_FLAG_USER))
> > +		return fault;
> > +
> > +	vma = lock_vma_under_rcu(current->mm, vmf->real_address);
> > +	if (!vma)
> > +		return fault;
> > +
> > +	if (arch_vma_access_error(vma, vmf)) {
> > +		vma_end_read(vma);
> > +		return fault;
> > +	}
> > +
> > +	fault = handle_mm_fault(vma, vmf->real_address,
> > +				vmf->flags | FAULT_FLAG_VMA_LOCK, vmf->regs);
> > +
> > +	if (!(fault & (VM_FAULT_RETRY | VM_FAULT_COMPLETED)))
> > +		vma_end_read(vma);
>
> Could you please explain how vma_end_read() call could be conditional?

The check is added for swap and userfault, see

https://lkml.kernel.org/r/20230630211957.1341547-4-surenb@xxxxxxxxxx
> > +
> > +	if (fault & VM_FAULT_RETRY)
> > +		count_vm_vma_lock_event(VMA_LOCK_RETRY);
> > +	else
> > +		count_vm_vma_lock_event(VMA_LOCK_SUCCESS);
> > +
> > +	return fault;
> > +}
> > +
> >   #endif /* CONFIG_PER_VMA_LOCK */
> >   
> >   #ifndef __PAGETABLE_P4D_FOLDED