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.
