On Mon, Jan 9, 2023 at 9:55 PM Suren Baghdasaryan <surenb@xxxxxxxxxx> wrote: > Due to the possibility of handle_userfault dropping mmap_lock, avoid fault > handling under VMA lock and retry holding mmap_lock. This can be handled > more gracefully in the future. > > Signed-off-by: Suren Baghdasaryan <surenb@xxxxxxxxxx> > Suggested-by: Peter Xu <peterx@xxxxxxxxxx> > --- > mm/memory.c | 7 +++++++ > 1 file changed, 7 insertions(+) > > diff --git a/mm/memory.c b/mm/memory.c > index 20806bc8b4eb..12508f4d845a 100644 > --- a/mm/memory.c > +++ b/mm/memory.c > @@ -5273,6 +5273,13 @@ struct vm_area_struct *lock_vma_under_rcu(struct mm_struct *mm, > if (!vma->anon_vma) > goto inval; > > + /* > + * Due to the possibility of userfault handler dropping mmap_lock, avoid > + * it for now and fall back to page fault handling under mmap_lock. > + */ > + if (userfaultfd_armed(vma)) > + goto inval; This looks racy wrt concurrent userfaultfd_register(). I think you'll want to do the userfaultfd_armed(vma) check _after_ locking the VMA, and ensure that the userfaultfd code write-locks the VMA before changing the __VM_UFFD_FLAGS in vma->vm_flags. > if (!vma_read_trylock(vma)) > goto inval; > > -- > 2.39.0 >
