Thanks for the detailed report Dimitris! I've CCed the MM mailing list and some folks who work on userfaultfd. I took a look at this today, but I haven't quite come up with a solution. I thought it might be as easy as changing userfaultfd_release() to set released *after* taking the lock. But no such luck, the ordering is what it is to deal with another subtle case: WRITE_ONCE(ctx->released, true); if (!mmget_not_zero(mm)) goto wakeup; /* * Flush page faults out of all CPUs. NOTE: all page faults * must be retried without returning VM_FAULT_SIGBUS if * userfaultfd_ctx_get() succeeds but vma->vma_userfault_ctx * changes while handle_userfault released the mmap_lock. So * it's critical that released is set to true (above), before * taking the mmap_lock for writing. */ mmap_write_lock(mm); I think perhaps the right thing to do is to have handle_userfault() release mmap_lock when it returns VM_FAULT_NOPAGE, and to have GUP deal with that appropriately? But, some investigation is required to be sure that's okay to do in the other non-GUP ways we can end up in handle_userfault().
