On Fri, Nov 20, 2020 at 1:36 PM Jens Axboe <axboe@xxxxxxxxx> wrote:
>
> I don't disagree with you on that. I've been a bit gun shy on touching
> the VFS side of things, but this one isn't too bad. I hacked up a patch
> that allows io_uring to do LOOKUP_RCU and a quick test seems to indicate
> it's fine. On top of that, we just propagate the error if we do fail and
> get rid of that odd retry loop.
Ok, this looks better to me (but is obviously not 5.10 material).
That said, I think I'd prefer to keep 'struct nameidata' internal to
just fs/namei.c, and maybe we can just expert that
struct nameidata nd;
set_nameidata(&nd, req->open.dfd, req->open.filename);
file = path_openat(&nd, &op, op.lookup_flags | LOOKUP_RCU);
restore_nameidata();
return filp == ERR_PTR(-ECHILD) ? -EAGAIN : filp;
as a helper from namei.c instead? Call it "do_filp_open_rcu()" or something?
That "force_nonblock" test seems a bit off, though. Why is that RCU
case only done when "!force_nonblock"? It would seem that if
force_nonblock is set, you want to do this too?
Al? You can see the background on lkml, but basically io_uring wants
to punt file open to a kernel thread, except if it can just be done
directly without blocking (which is pretty much that RCU lookup case).
And the thing that triggered this is that /proc/self/ can only be done
directly - not in a kernel thread. So the RCU case actually ends up
being interesting in that it would handl those things.
Linus
