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

Hao Xu
2023-08-29T07:42:08Z

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 = &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.

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);
}