Re: [PATCH] userfaultfd: Write protect when virtual memory range has no page table entry

[Bui Quang Minh]

On Mon, Mar 22, 2021 at 03:00:37PM +0200, Mike Rapoport wrote:
> On Mon, Mar 22, 2021 at 11:14:37AM +0100, Michal Hocko wrote:
> > Le'ts Andrea and Mike
> > 
> > On Fri 19-03-21 22:24:28, Bui Quang Minh wrote:
> > > userfaultfd_writeprotect() use change_protection() to clear write bit in
> > > page table entries (pte/pmd). So, later write to this virtual address
> > > range causes a page fault, which is then handled by userspace program.
> > > However, change_protection() has no effect when there is no page table
> > > entries associated with that virtual memory range (a newly mapped memory
> > > range). As a result, later access to that memory range causes allocating a
> > > page table entry with write bit still set (due to VM_WRITE flag in
> > > vma->vm_flags).
> > > 
> > > Add checks for VM_UFFD_WP in vma->vm_flags when allocating new page table
> > > entry in missing page table entry page fault path.
> > 
> > From the above it is not really clear whether this is a usability
> > problem or a bug of the interface.
> 
> I'd say it's usability/documentation clarity issue. 
> Userspace can register an area with
> 
> 	UFFDIO_REGISTER_MODE_MISSING | UFFDIO_REGISTER_MODE_WP
> 
> and then it will be notified either when page table has no entry for a
> virtual address or when there is a write to a write protected address.

Yes, you are right. I saw a patch from Peter to linux-man and saw that

	"When there is only UFFDIO_REGISTER_MODE_WP registered, the userspace
	will not receive any message when a missing page is written"

It's my mistake that I didn't look at the documentation carefully when playing
around.

Thanks,
Quang Minh.