On Mon 22-03-21 14:49:35, Michal Hocko wrote:

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

quoted

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.

Btw. Andrew the patch still seems to be in mmotm. Do you plan to keep it
there?

-- 
Michal Hocko
SUSE Labs