On Fri, Jan 26, 2018 at 11:17:29PM +0000, Al Viro wrote:
> On Fri, Jan 26, 2018 at 11:13:51PM +0000, Al Viro wrote:
>
> > Erm... And just what is protecting the list here?
Addressed in v2/v3, sorry about that.
> > > static void fdatawrite_one_bdev(struct block_device *bdev, void *arg)
> > > @@ -107,12 +138,18 @@ static void fdatawait_one_bdev(struct block_device *bdev, void *arg)
> > > */
> > > SYSCALL_DEFINE0(sync)
> > > {
> > > - int nowait = 0, wait = 1;
> > > + struct sb_sync arg = {
> > > + .mnt_ns = current->nsproxy->mnt_ns,
> > > + };
> > > +
> > > + if (arg.mnt_ns == init_task.nsproxy->mnt_ns)
> > > + arg.mnt_ns = NULL;
> > >
> > > wakeup_flusher_threads(WB_REASON_SYNC);
> > > - iterate_supers(sync_inodes_one_sb, NULL);
> > > - iterate_supers(sync_fs_one_sb, &nowait);
> > > - iterate_supers(sync_fs_one_sb, &wait);
> > > + iterate_supers(sync_inodes_one_sb, &arg);
> > > + iterate_supers(sync_fs_one_sb, &arg);
> > > + arg.wait = 1;
> > > + iterate_supers(sync_fs_one_sb, &arg);
> >
> > So now sync() includes O(total vfsmounts on the system) walking the lists, no
> > matter what
This bit I thought about, and the only alternative I could come up with
was walking mnt_ns->list which makes it O(vfsmounts in the namespace)
which is at least capped at sysctl_mount_max. The downside of that is
that we have to track duplicates to handle super blocks mounted in more
than one place. And, of course, it doesn't address your other points.
> > *and* in a situation when a lazy-unmounted filesystem is held active
> > by an opened file sync(2) won't touch that filesystem. Unless done in the
> > magical namespace init(8) happens to run in.
Hm, so we could always sync the filesystem if s_mounts is empty to cover
this.
> BTW, if your process happens to have inherited an opened file from parent,
> then unshares the namespace and unmounts the filesystem that file came
> from, sync(2) won't affect the writes on that descriptor.
But for this, I don't have any good ideas. Either we track these things
in the hot path or do something O(number of open fds). Do you have any
suggestions?
