On Tue, Jan 17, 2023 at 12:36 PM Jann Horn <jannh@xxxxxxxxxx> wrote: > > On Tue, Jan 17, 2023 at 8:51 PM Jann Horn <jannh@xxxxxxxxxx> wrote: > > 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, > > I still think this change is needed... Yes, I think you are right. I'll move the check after locking the VMA. Thanks! > > > and ensure that the userfaultfd code write-locks the VMA before > > changing the __VM_UFFD_FLAGS in vma->vm_flags. > > Ah, but now I see you already took care of this half of the issue with > the reset_vm_flags() change in > https://lore.kernel.org/linux-mm/20230109205336.3665937-16-surenb@xxxxxxxxxx/ > . > > > > > if (!vma_read_trylock(vma)) > > > goto inval; > > > > > > -- > > > 2.39.0 > > >
