On Thu, 14 Dec 2017 10:20:18 +0800 "Yan, Zheng" <z...@redhat.com> wrote:
> >> + /* > >> + * If the fault happens during write_iter() copies data from > >> + * userspace, filesystem may have set current->journal_info. > >> + * If the userspace memory is mapped to a file on another > >> + * filesystem, fault handler of the later filesystem may want > >> + * to access/modify current->journal_info. > >> + */ > >> + current->journal_info = NULL; > >> ret = vma->vm_ops->fault(vmf); > >> + /* Restore original journal_info */ > >> + current->journal_info = old_journal_info; > >> if (unlikely(ret & (VM_FAULT_ERROR | VM_FAULT_NOPAGE | VM_FAULT_RETRY | > >> VM_FAULT_DONE_COW))) > >> return ret; > > > > Can you explain why you chose these two sites? Rather than, for > > example, way up in handle_mm_fault()? > > I think they are the only two places that code can enter another filesystem hm. Maybe. At this point in time. I'm feeling that doing the save/restore at the highest level is better. It's cheap. > > > > It's hard to believe that a fault handler will alter ->journal_info if > > it is handling a read fault, so perhaps we only need to do this for a > > write fault? Although such an optimization probably isn't worthwhile. > > The whole thing is only about three instructions. > > ceph uses current->journal_info for both read/write operations. I think btrfs > also read current->journal_info during read-only operation. (I mentioned this > in my previous reply) Quite a lot of filesystems use ->journal_info. Arguably it should be the fs's responsibility to restore the old journal_info value after having used it. But that's a ton of changes :(
