On Tue, Feb 12, 2019 at 10:56:09AM +0800, Peter Xu wrote: > The idea comes from the upstream discussion between Linus and Andrea: > > https://lkml.org/lkml/2017/10/30/560 > > A summary to the issue: there was a special path in handle_userfault() > in the past that we'll return a VM_FAULT_NOPAGE when we detected > non-fatal signals when waiting for userfault handling. We did that by > reacquiring the mmap_sem before returning. However that brings a risk > in that the vmas might have changed when we retake the mmap_sem and > even we could be holding an invalid vma structure. > > This patch removes the risk path in handle_userfault() then we will be > sure that the callers of handle_mm_fault() will know that the VMAs > might have changed. Meanwhile with previous patch we don't lose > responsiveness as well since the core mm code now can handle the > nonfatal userspace signals quickly even if we return VM_FAULT_RETRY. > > Suggested-by: Andrea Arcangeli <aarcange@xxxxxxxxxx> > Suggested-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> > Signed-off-by: Peter Xu <peterx@xxxxxxxxxx> Reviewed-by: Jérôme Glisse <jglisse@xxxxxxxxxx> > --- > fs/userfaultfd.c | 24 ------------------------ > 1 file changed, 24 deletions(-) > > diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c > index 89800fc7dc9d..b397bc3b954d 100644 > --- a/fs/userfaultfd.c > +++ b/fs/userfaultfd.c > @@ -514,30 +514,6 @@ vm_fault_t handle_userfault(struct vm_fault *vmf, unsigned long reason) > > __set_current_state(TASK_RUNNING); > > - if (return_to_userland) { > - if (signal_pending(current) && > - !fatal_signal_pending(current)) { > - /* > - * If we got a SIGSTOP or SIGCONT and this is > - * a normal userland page fault, just let > - * userland return so the signal will be > - * handled and gdb debugging works. The page > - * fault code immediately after we return from > - * this function is going to release the > - * mmap_sem and it's not depending on it > - * (unlike gup would if we were not to return > - * VM_FAULT_RETRY). > - * > - * If a fatal signal is pending we still take > - * the streamlined VM_FAULT_RETRY failure path > - * and there's no need to retake the mmap_sem > - * in such case. > - */ > - down_read(&mm->mmap_sem); > - ret = VM_FAULT_NOPAGE; > - } > - } > - > /* > * Here we race with the list_del; list_add in > * userfaultfd_ctx_read(), however because we don't ever run > -- > 2.17.1 >
