Re: [PATCH 02/11] xfs: add NOWAIT semantics for readdir

[Hao Xu]

On 8/28/23 04:44, Matthew Wilcox wrote:
> On Sun, Aug 27, 2023 at 09:28:26PM +0800, Hao Xu wrote:
> > +++ b/fs/xfs/libxfs/xfs_da_btree.c
> > @@ -2643,16 +2643,32 @@ xfs_da_read_buf(
> >   	struct xfs_buf_map	map, *mapp = ↦
> >   	int			nmap = 1;
> >   	int			error;
> > +	int			buf_flags = 0;
> >   
> >   	*bpp = NULL;
> >   	error = xfs_dabuf_map(dp, bno, flags, whichfork, &mapp, &nmap);
> >   	if (error || !nmap)
> >   		goto out_free;
> >   
> > +	/*
> > +	 * NOWAIT semantics mean we don't wait on the buffer lock nor do we
> > +	 * issue IO for this buffer if it is not already in memory. Caller will
> > +	 * retry. This will return -EAGAIN if the buffer is in memory and cannot
> > +	 * be locked, and no buffer and no error if it isn't in memory.  We
> > +	 * translate both of those into a return state of -EAGAIN and *bpp =
> > +	 * NULL.
> > +	 */
>
> I would not include this comment.

No strong comment here, since this patch is mostly from Dave, it's
better if Dave can ack this.

> > +	if (flags & XFS_DABUF_NOWAIT)
> > +		buf_flags |= XBF_TRYLOCK | XBF_INCORE;
> >   	error = xfs_trans_read_buf_map(mp, tp, mp->m_ddev_targp, mapp, nmap, 0,
> >   			&bp, ops);
>
> what tsting did you do with this?  Because you don't actually _use_
> buf_flags anywhere in this patch (presumably they should be the
> sixth argument to xfs_trans_read_buf_map() instead of 0).  So I can only
> conclude that either you didn't test, or your testing was inadequate.


The tests I've done are listed in the cover-letter, this one is missed, 
the tricky place is it's hard to get this kind of mistake since it runs
well without nowait logic...I'll fix it in next version.

> >   	if (error)
> >   		goto out_free;
> > +	if (!bp) {
> > +		ASSERT(flags & XFS_DABUF_NOWAIT);
>
> I don't think this ASSERT is appropriate.
>
> > @@ -391,10 +401,17 @@ xfs_dir2_leaf_getdents(
> >   				bp = NULL;
> >   			}
> >   
> > -			if (*lock_mode == 0)
> > -				*lock_mode = xfs_ilock_data_map_shared(dp);
> > +			if (*lock_mode == 0) {
> > +				*lock_mode =
> > +					xfs_ilock_data_map_shared_generic(dp,
> > +					ctx->flags & DIR_CONTEXT_F_NOWAIT);
> > +				if (!*lock_mode) {
> > +					error = -EAGAIN;
> > +					break;
> > +				}
> > +			}
>
> 'generic' doesn't seem like a great suffix to mean 'takes nowait flag'.
> And this is far too far indented.
>
> 			xfs_dir2_lock(dp, ctx, lock_mode);
>
> with:
>
> STATIC void xfs_dir2_lock(struct xfs_inode *dp, struct dir_context *ctx,
> 		unsigned int lock_mode)
> {
> 	if (*lock_mode)
> 		return;
> 	if (ctx->flags & DIR_CONTEXT_F_NOWAIT)
> 		return xfs_ilock_data_map_shared_nowait(dp);
> 	return xfs_ilock_data_map_shared(dp);
> }
>
> ... which I think you can use elsewhere in this patch (reformat it to
> XFS coding style, of course).  And then you don't need
> xfs_ilock_data_map_shared_generic().

How about rename xfs_ilock_data_map_shared() to 
xfs_ilock_data_map_block() and rename 
xfs_ilock_data_map_shared_generic() to xfs_ilock_data_map_shared()?

STATIC void xfs_ilock_data_map_shared(struct xfs_inode *dp, struct 
dir_context *ctx, unsigned int lock_mode)
{
 	if (*lock_mode)
 		return;
 	if (ctx->flags & DIR_CONTEXT_F_NOWAIT)
 		return xfs_ilock_data_map_shared_nowait(dp);
 	return xfs_ilock_data_map_shared_block(dp);
}