On Mon 22-03-21 15:00:37, Mike Rapoport wrote:

On Mon, Mar 22, 2021 at 11:14:37AM +0100, Michal Hocko wrote:

quoted

Le'ts Andrea and Mike

On Fri 19-03-21 22:24:28, Bui Quang Minh wrote:

quoted

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.

Thanks for the clarification! I have suspected this to be the case but
I am not really familiar with the interface to have any strong statement
here. Maybe we want to document this explicitly.
-- 
Michal Hocko
SUSE Labs