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

Matthew Wilcox
2023-08-27T20:45:56Z

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().