On Fri, Sep 28, 2018 at 04:19:03PM +0800, Huang, Ying wrote: > Daniel Jordan <daniel.m.jordan@xxxxxxxxxx> writes: > > One way is to change > > copy_one_pte's return to int so we can just pass the error code back to > > copy_pte_range so it knows whether to try adding the continuation. > > There may be even more problems. After add_swap_count_continuation(), > copy_one_pte() will be retried, and the CPU may hang with dead loop. That's true, it would do that. > But before the changes in this patchset, the behavior is, > __swap_duplicate() return an error that isn't -ENOMEM, such as -EEXIST. > Then copy_one_pte() would thought the operation has been done > successfully, and go to call set_pte_at(). This will cause the system > state become inconsistent, and the system may panic or hang somewhere > later. > > So per my understanding, if we thought page table corruption isn't a > real problem (that is, __swap_duplicate() will never return e.g. -EEXIST > if copied by copy_one_pte() indirectly), both the original and the new > code should be OK. > > If we thought it is a real problem, we need to fix the original code and > keep it fixed in the new code. Do you agree? Yes, if it was a real problem, which seems less and less the case the more I stare at this. > There's several ways to fix the problem. But the page table shouldn't > be corrupted in practice, unless there's some programming error. So I > suggest to make it as simple as possible via adding, > > VM_BUG_ON(error != -ENOMEM); > > in swap_duplicate(). > > Do you agree? Yes, I'm ok with that, adding in -ENOTDIR along with it. The error handling in __swap_duplicate (before this series) still leaves something to be desired IMHO. Why all the different returns when callers ignore them or only specifically check for -ENOMEM or -EEXIST? Could maybe stand a cleanup, but outside this series.
