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

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Date: Sun, 27 Aug 2023 21:44:53 +0100
From: Matthew Wilcox <willy@...radead.org>
To: Hao Xu <hao.xu@...ux.dev>
Cc: io-uring@...r.kernel.org, Jens Axboe <axboe@...nel.dk>,
	Dominique Martinet <asmadeus@...ewreck.org>,
	Pavel Begunkov <asml.silence@...il.com>,
	Christian Brauner <brauner@...nel.org>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Stefan Roesch <shr@...com>, Clay Harris <bugs@...ycon.org>,
	Dave Chinner <david@...morbit.com>,
	"Darrick J . Wong" <djwong@...nel.org>,
	linux-fsdevel@...r.kernel.org, linux-xfs@...r.kernel.org,
	linux-ext4@...r.kernel.org, linux-cachefs@...hat.com,
	ecryptfs@...r.kernel.org, linux-nfs@...r.kernel.org,
	linux-unionfs@...r.kernel.org, bpf@...r.kernel.org,
	netdev@...r.kernel.org, linux-s390@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-block@...r.kernel.org,
	linux-btrfs@...r.kernel.org, codalist@...a.cs.cmu.edu,
	linux-f2fs-devel@...ts.sourceforge.net, cluster-devel@...hat.com,
	linux-mm@...ck.org, linux-nilfs@...r.kernel.org,
	devel@...ts.orangefs.org, linux-cifs@...r.kernel.org,
	samba-technical@...ts.samba.org, linux-mtd@...ts.infradead.org,
	Wanpeng Li <wanpengli@...cent.com>
Subject: Re: [PATCH 02/11] xfs: add NOWAIT semantics for readdir

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 = &map;
>  	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.

> +	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.

>  	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().