Re: [PATCH 3/5] io_uring: add support for getdents

[Christian Brauner <brauner@kernel.org>]

On Thu, Jul 27, 2023 at 07:51:19PM +0800, Hao Xu wrote:
> On 7/26/23 23:00, Christian Brauner wrote:
> > On Tue, Jul 18, 2023 at 09:21:10PM +0800, Hao Xu wrote:
> > > From: Hao Xu <howeyxu@tencent.com>
> > > 
> > > This add support for getdents64 to io_uring, acting exactly like the
> > > syscall: the directory is iterated from it's current's position as
> > > stored in the file struct, and the file's position is updated exactly as
> > > if getdents64 had been called.
> > > 
> > > For filesystems that support NOWAIT in iterate_shared(), try to use it
> > > first; if a user already knows the filesystem they use do not support
> > > nowait they can force async through IOSQE_ASYNC in the sqe flags,
> > > avoiding the need to bounce back through a useless EAGAIN return.
> > > 
> > > Co-developed-by: Dominique Martinet <asmadeus@codewreck.org>
> > > Signed-off-by: Dominique Martinet <asmadeus@codewreck.org>
> > > Signed-off-by: Hao Xu <howeyxu@tencent.com>
> > > ---
> > >   include/uapi/linux/io_uring.h |  7 +++++
> > >   io_uring/fs.c                 | 55 +++++++++++++++++++++++++++++++++++
> > >   io_uring/fs.h                 |  3 ++
> > >   io_uring/opdef.c              |  8 +++++
> > >   4 files changed, 73 insertions(+)
> > > 
> > > diff --git a/include/uapi/linux/io_uring.h b/include/uapi/linux/io_uring.h
> > > index 36f9c73082de..b200b2600622 100644
> > > --- a/include/uapi/linux/io_uring.h
> > > +++ b/include/uapi/linux/io_uring.h
> > > @@ -65,6 +65,7 @@ struct io_uring_sqe {
> > >   		__u32		xattr_flags;
> > >   		__u32		msg_ring_flags;
> > >   		__u32		uring_cmd_flags;
> > > +		__u32		getdents_flags;
> > >   	};
> > >   	__u64	user_data;	/* data to be passed back at completion time */
> > >   	/* pack this to avoid bogus arm OABI complaints */
> > > @@ -235,6 +236,7 @@ enum io_uring_op {
> > >   	IORING_OP_URING_CMD,
> > >   	IORING_OP_SEND_ZC,
> > >   	IORING_OP_SENDMSG_ZC,
> > > +	IORING_OP_GETDENTS,
> > >   	/* this goes last, obviously */
> > >   	IORING_OP_LAST,
> > > @@ -273,6 +275,11 @@ enum io_uring_op {
> > >    */
> > >   #define SPLICE_F_FD_IN_FIXED	(1U << 31) /* the last bit of __u32 */
> > > +/*
> > > + * sqe->getdents_flags
> > > + */
> > > +#define IORING_GETDENTS_REWIND	(1U << 0)
> > > +
> > >   /*
> > >    * POLL_ADD flags. Note that since sqe->poll_events is the flag space, the
> > >    * command flags for POLL_ADD are stored in sqe->len.
> > > diff --git a/io_uring/fs.c b/io_uring/fs.c
> > > index f6a69a549fd4..480f25677fed 100644
> > > --- a/io_uring/fs.c
> > > +++ b/io_uring/fs.c
> > > @@ -47,6 +47,13 @@ struct io_link {
> > >   	int				flags;
> > >   };
> > > +struct io_getdents {
> > > +	struct file			*file;
> > > +	struct linux_dirent64 __user	*dirent;
> > > +	unsigned int			count;
> > > +	int				flags;
> > > +};
> > > +
> > >   int io_renameat_prep(struct io_kiocb *req, const struct io_uring_sqe *sqe)
> > >   {
> > >   	struct io_rename *ren = io_kiocb_to_cmd(req, struct io_rename);
> > > @@ -291,3 +298,51 @@ void io_link_cleanup(struct io_kiocb *req)
> > >   	putname(sl->oldpath);
> > >   	putname(sl->newpath);
> > >   }
> > > +
> > > +int io_getdents_prep(struct io_kiocb *req, const struct io_uring_sqe *sqe)
> > > +{
> > > +	struct io_getdents *gd = io_kiocb_to_cmd(req, struct io_getdents);
> > > +
> > > +	if (READ_ONCE(sqe->off) != 0)
> > > +		return -EINVAL;
> > > +
> > > +	gd->dirent = u64_to_user_ptr(READ_ONCE(sqe->addr));
> > > +	gd->count = READ_ONCE(sqe->len);
> > > +
> > > +	return 0;
> > > +}
> > > +
> > > +int io_getdents(struct io_kiocb *req, unsigned int issue_flags)
> > > +{
> > > +	struct io_getdents *gd = io_kiocb_to_cmd(req, struct io_getdents);
> > > +	struct file *file = req->file;
> > > +	unsigned long getdents_flags = 0;
> > > +	bool force_nonblock = issue_flags & IO_URING_F_NONBLOCK;
> > 
> > Hm, I'm not sure what exactly the rules are for IO_URING_F_NONBLOCK.
> > But to point this out:
> > 
> > vfs_getdents()
> > -> iterate_dir()
> >     {
> >          if (shared)
> >                  res = down_read_killable(&inode->i_rwsem);
> >          else
> >                  res = down_write_killable(&inode->i_rwsem);
> >     }
> > 
> > which means you can still end up sleeping here before you go into a
> > filesystem that does actually support non-waiting getdents. So if you
> > have concurrent operations that grab inode lock (touch, mkdir etc) you
> > can end up sleeping here.
> > 
> > Is that intentional or an oversight? If the former can someone please
> > explain the rules and why it's fine in this case?
> 
> I actually saw this semaphore, and there is another xfs lock in
> file_accessed
>   --> touch_atime
>     --> inode_update_time
>       --> inode->i_op->update_time == xfs_vn_update_time
> 
> Forgot to point them out in the cover-letter..., I didn't modify them
> since I'm not very sure about if we should do so, and I saw Stefan's
> patchset didn't modify them too.
> 
> My personnal thinking is we should apply trylock logic for this
> inode->i_rwsem. For xfs lock in touch_atime, we should do that since it
> doesn't make sense to rollback all the stuff while we are almost at the
> end of getdents because of a lock.

That manoeuvres around the problem. Which I'm slightly more sensitive
too as this review is a rather expensive one.

Plus, it seems fixable in at least two ways:

For both we need to be able to tell the filesystem that a nowait atime
update is requested. Simple thing seems to me to add a S_NOWAIT flag to
file_time_flags and passing that via i_op->update_time() which already
has a flag argument. That would likely also help kiocb_modified().

file_accessed()
-> touch_atime()
   -> inode_update_time()
      -> i_op->update_time == xfs_vn_update_time()

Then we have two options afaict:

(1) best-effort atime update

file_accessed() already has the builtin assumption that updating atime
might fail for other reasons - see the comment in there. So it is
somewhat best-effort already.

(2) move atime update before calling into filesystem

If we want to be sure that access time is updated when a readdir request
is issued through io_uring then we need to have file_accessed() give a
return value and expose a new helper for io_uring or modify
vfs_getdents() to do something like:

vfs_getdents()
{
	if (nowait)
		down_read_trylock()

	if (!IS_DEADDIR(inode)) {
		ret = file_accessed(file);
		if (ret == -EAGAIN)
			goto out_unlock;

		f_op->iterate_shared()
	}
}

It's not unprecedented to do update atime before the actual operation
has been done afaict. That's already the case in xfs_file_write_checks()
which is called before anything is written. So that seems ok.

Does any of these two options work for the xfs maintainers and Jens?