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

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Message-ID: <ca10040f-b7fa-7c43-1c89-6706d13b2747@linux.dev>
Date: Tue, 29 Aug 2023 15:41:43 +0800
From: Hao Xu <hao.xu@...ux.dev>
To: Matthew Wilcox <willy@...radead.org>
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 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);
}