On Wed, Feb 01, 2023 at 12:41:37PM +0900, David Stevens wrote: > From: David Stevens <stevensd@xxxxxxxxxxxx> > > Collapsing memory in a vma that has an armed userfaultfd results in > zero-filling any missing pages, which breaks user-space paging for those > filled pages. Avoid khugepage bypassing userfaultfd by not collapsing > pages in shmem reached via scanning a vma with an armed userfaultfd if > doing so would zero-fill any pages. Could you elaborate on the failure? Will zero-filling the page prevent userfaultfd from catching future access? A test-case would help a lot. And what prevents the same pages be filled (with zeros or otherwise) via write(2) bypassing VMA checks? I cannot immediately see it. BTW, there's already a check that prevent establishing PMD in the place if VM_UFFD_WP is set. Maybe just an update of the check in retract_page_tables() from userfaultfd_wp() to userfaultfd_armed() would be enough? I have very limited understanding of userfaultfd(). Sorry in advance for stupid questions. -- Kiryl Shutsemau / Kirill A. Shutemov
